I have the same problem, also with archlinux 64-bit. The error seems to appear with lib32-glibc 2.18, if I downgrade to 2.17, steam works again.

Output:
Running Steam on arch rolling 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(1376442816_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/crash_20130815165653_1.dmp
/home/user/.local/share/Steam/steam.sh: line 704: 11174 Segmentation fault (core dumped) $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
Installing bootstrap /home/user/.local/share/Steam/bootstrap.tar.xz
Running Steam on arch rolling 64-bit
STEAM_RUNTIME has been set by the user to: /home/user/.local/share/Steam/ubuntu12_32/steam-runtime
Installing breakpad exception handler for appid(steam)/version(1376442816_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/crash_20130815165653_1.dmp
/home/user/.local/share/Steam/steam.sh: line 704: 11259 Segmentation fault (core dumped) $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-9f19d127-d29a-43f9-b4e9-c4c762130815
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-196acc44-58e0-4565-9703-da2822130815

Yes, downgrading lib32-glibc works for me aswell. Looking through /var/log/pacman.log, it seems that package was also updated yesterday. Don't know if this means there's a problem with lib32-glibc, Arch Linux, or Steam.

I'd just like to mention that if I don't use Steam's Runtime but my system's librareis, I also get a Segfault with nVidia Drivers on Steam startup. Downgrading lib32-glibc to 2.17 fixes the issue.

If I do use steam runtime, then Steam starts correctly, but for example DoD:Source will segfaults right before the menu appears. Again, downgrading lib32-glibc fixes the issue.

I'm using the latest nvidia-drivers 325.15, on linux 3.10.6

Downgrading to lib32-glibc 2.17 fixed seg. fault, but steam doesn't launch anyway:

$ steam
Running Steam on arch rolling 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(1374875626_client)
Installing breakpad exception handler for appid(steam)/version(1374875626_client)
Installing breakpad exception handler for appid(steam)/version(1374875626_client)
unlinked 0 orphaned pipes
removing stale semaphore last operated on by process 717 with name 0eBlobRegistryMutex_8F29413B20C00B3F50652D321779A1B9
removing stale semaphore last operated on by process 717 with name 0eBlobRegistrySignal_8F29413B20C00B3F50652D321779A1B9
removing stale semaphore last operated on by process 717 with name 0emSteamEngineInstance
removing stale semaphore last operated on by process 717 with name 0eSteamEngineLock
Installing breakpad exception handler for appid(steam)/version(1374875626_client)
Fontconfig error: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 70: non-double matrix element
Fontconfig error: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 70: non-double matrix element
Fontconfig warning: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 78: saw unknown, expected number
Steam: An X Error occurred
X Error of failed request: BadRequest (invalid request code or no such operation)
Major opcode of failed request: 155 (ATIFGLEXTENSION)
Minor opcode of failed request: 66
Serial number of failed request: 35
xerror_handler: X failed, continuing
Steam: An X Error occurred
X Error of failed request: BadRequest (invalid request code or no such operation)
Major opcode of failed request: 154 (GLX)
Minor opcode of failed request: 19 (X_GLXQueryServerString)
Serial number of failed request: 66
xerror_handler: X failed, continuing
Steam: An X Error occurred
X Error of failed request: BadRequest (invalid request code or no such operation)
Major opcode of failed request: 154 (GLX)
Minor opcode of failed request: 19 (X_GLXQueryServerString)
Serial number of failed request: 95
xerror_handler: X failed, continuing
Steam: An X Error occurred
X Error of failed request: BadRequest (invalid request code or no such operation)
Major opcode of failed request: 154 (GLX)
Minor opcode of failed request: 14 (X_GLXGetVisualConfigs)
Serial number of failed request: 124
xerror_handler: X failed, continuing
$

Had the same issue, but it wasn't contained to Steam so I took a look around the Arch bugtracker and found https://bugs.archlinux.org/task/36556.
Seems like this is fixed with 2.18-2, and for those not in [testing], the rebuilding with a different optimization than -O2 fix seems to have worked for me as well.

@ToostInc , was that for glibc or lib32-glibc? I could fix the issue by downgrading lib32-glibc, not glibc.

In fact, trying with glibc-2.18-2 I still get the issue. I'm waiting for lib32-glibc-2.18-2 to appear in [Testing] to re-test.

lib32-glibc-2.18-2 available in Multilib-Testing, fixed any issue I had with Steam. Anyone can confirm?

Confirmed, works now. Haven't rebooted though, don't know if needed. Upgraded and restarted steam and it works now.

@jpleau, the Arch bugreport was for glibc, but rebuilding 2.18-1 or upgrading to 2.18-2 fix the problems with lib32-glibc as well.
This works for Arch Linux at least, but not sure anyone in this bug report is running anything different anyway.

And @KlavKalashj, linux users don't really need to reboot, unless there's a kernel or kernel module upgrade.

UPDATE: 2.18-2 was just pushed to multilib.