Hello,
to reproduce the crash, start Steam, search X3 in the search field, click on X3: Terran Conflict to visualise the description (I didn't buy the game). My Steam just crashes.

I started Steam from the console to have more info:

Here is the core dump I uploaded:
https://dl.dropboxusercontent.com/u/11057287/crash_20130601224728_1.dmp

Here is the log with the crash:

esteban@esteban-laptop:~$ steam
Running Steam on ubuntu 13.04 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
unlinked 0 orphaned pipes
Gtk-Message: Failed to load module "overlay-scrollbar"

(steam:9495): Gtk-WARNING **: Unable to locate theme engine in module_path: "oxygen-gtk",
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
[0601/224640:WARNING:proxy_service.cc(646)] PAC support disabled because there is no system implementation
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
Generating new string page texture 2: 48x256, total string texture memory is 49.15 KB
Generating new string page texture 3: 256x256, total string texture memory is 311.30 KB
Installing breakpad exception handler for appid(steam)/version(1369961861_client)

(steam:9495): GLib-GIO-WARNING **: g_simple_async_result_complete() called from wrong context!

(steam:9495): GLib-GIO-WARNING **: g_simple_async_result_complete() called from wrong context!
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
roaming config store loaded successfully - 777 bytes.
migrating temporary roaming config store
`menu_proxy_module_load': /home/esteban/.local/share/Steam/ubuntu12_32/steam: undefined symbol: menu_proxy_module_load

(steam:9495): Gtk-WARNING **: Failed to load type module: (null)

Adding license for package 0
Adding license for package 1995
Adding license for package 2008
Adding license for package 6143
Adding license for package 11707
Adding license for package 11795
Adding license for package 14254
Adding license for package 17915
Adding license for package 25643
Adding license for package 27919
Adding license for package 27945
Adding license for package 28208
Adding license for package 28221
Installing breakpad exception handler for appid(steam)/version(1369961861_client)
ExecCommandLine: "/home/esteban/.local/share/Steam/ubuntu12_32/steam"
Generating new string page texture 73: 128x256, total string texture memory is 442.37 KB
Generating new string page texture 74: 128x256, total string texture memory is 131.07 KB
Generating new string page texture 75: 32x256, total string texture memory is 475.14 KB
Generating new string page texture 76: 64x256, total string texture memory is 540.67 KB

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:9495): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.
CAPIJobRequestUserStats - Server response failed 2
System startup time: 10.66 seconds
Running Steam on ubuntu 13.04 64-bit
STEAM_RUNTIME has been set by the user to: /home/esteban/.local/share/Steam/ubuntu12_32/steam-runtime
ExecCommandLine: "/home/esteban/.steam/root/ubuntu12_32/steam steam://open/driverhelperready"
ExecSteamURL: "steam://open/driverhelperready"
[0601/224727:WARNING:backend_impl.cc(1930)] Destroying invalid entry.
[0601/224728:ERROR:reference_audio_renderer.cc(45)] Not implemented reached in virtual void media::ReferenceAudioRenderer::OnCreated(media::AudioOutputController*)
[0601/224728:ERROR:reference_audio_renderer.cc(49)] Not implemented reached in virtual void media::ReferenceAudioRenderer::OnPlaying(media::AudioOutputController*)
[0601/224728:ERROR:alsa_output.cc(684)] Failed querying delay: Input/output error
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/crash_20130601224728_1.dmp
/home/esteban/.local/share/Steam/steam.sh: line 704:  9495 Illegal instruction     (core dumped) $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-778d0bce-876a-44aa-a949-a21d52130601

The output from Steam->Help->System Information:

Processor Information:
    Vendor:  AuthenticAMD
    Speed: 2300 Mhz
    2 logical processors
    2 physical processors
    HyperThreading:  Unsupported
    FCMOV:  Supported
    SSE2:  Supported
    SSE3:  Supported
    SSSE3:  Unsupported
    SSE4a:  Supported
    SSE41:  Unsupported
    SSE42:  Unsupported
    
Network Information:
    Network Speed:  
    
Operating System Version:
    Ubuntu 13.04 (64 bit)
    Kernel Name:  Linux
    Kernel Version:  3.8.0-22-generic
    X Server Vendor:  The X.Org Foundation
    X Server Release:  11303000
    X Window Manager:  Compiz
    Steam Runtime Version:  steam-runtime-release-i386_2013-05-08
    
Video Card:
    Driver:  ATI Technologies Inc. AMD Radeon HD 6500M/5600/5700 Series

    Driver Version:  4.2.12002 Compatibility Profile Context 9.012
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 59 Hz
    VendorID:  0x1002
    DeviceID:  0x68c1
    Number of Monitors:  1
    Number of Logical Video Cards:  1
    Primary Display Resolution:  1366 x 768
    Desktop Resolution: 1366 x 768
    Primary Display Size: 13,54" x 7,60"  (15,51" diag)
                                            34,4cm x 19,3cm  (39,4cm diag)
    Primary VRAM Not Detected
    
Sound card:
    Audio device: Conexant CX20585
    
Memory:
    RAM:  3950 Mb
    
Miscellaneous:
    UI Language:  English
    LANG:  en_US.UTF-8
    Microphone:  Not set
    Total Hard Disk Space Available:  46805 Mb
    Largest Free Hard Disk Block:  37820 Mb
    
Installed software:
    
Recent Failure Reports:
    Sat Jun  1 20:46:02 2013 GMT: file ''/tmp/dumps/crash_20130601224552_1.dmp'', upload yes: ''CrashID=bp-458c5107-e989-45ec-92fa-0c3f62130601''
    Sat Jun  1 20:47:37 2013 GMT: file ''/tmp/dumps/crash_20130601224728_1.dmp'', upload yes: ''CrashID=bp-778d0bce-876a-44aa-a949-a21d52130601''

Thank you for your support!