I tried using the standard fglrx driver, but I have 2 monitors (2560x1600 and 1920x1080) that totals 4480 wide and exceeds the width allowed by the driver (max 4096). This caused the two screens to have an overlap (same image on both displays in the overlap region). With fglrx steam did start, although no games ran due to a different problem that might now be solved. The opens source video drivers had no problems with this size issue. So I decided to try the fglrx-amdcccle-experimental-9 driver. This solved the overlap problem (same as open source driver), but now steam will not run at all. If there are updates, a widow pops up while updates download, but after that I get a core dump. When I run again from the command line, the following is displayed:

john@transition$ steam steam://games
Installing breakpad exception handler for appid(steam)/version(1355957371_client)
SteamUpdateUI: An X Error occurred
X Error of failed request: BadRequest (invalid request code or no such operation)
Major opcode of failed request: 153 (GLX)
Minor opcode of failed request: 19 (X_GLXQueryServerString)
Serial number of failed request: 12
xerror_handler: X failed, continuing
SteamUpdateUI: An X Error occurred
X Error of failed request: BadRequest (invalid request code or no such operation)
Major opcode of failed request: 153 (GLX)
Minor opcode of failed request: 19 (X_GLXQueryServerString)
Serial number of failed request: 40
xerror_handler: X failed, continuing
SteamUpdateUI: An X Error occurred
X Error of failed request: BadRequest (invalid request code or no such operation)
Major opcode of failed request: 153 (GLX)
Minor opcode of failed request: 14 (X_GLXGetVisualConfigs)
Serial number of failed request: 68
xerror_handler: X failed, continuing
Looks like steam didn't shutdown cleanly, scheduling immediate update check
Installing breakpad exception handler for appid(steam)/version(1355957371_client)
Installing breakpad exception handler for appid(steam)/version(1355957371_client)
unlinked 0 orphaned pipes
removing stale semaphore last operated on by process 11513 with name 0eBlobRegistryMutex_A8F5D998AC40B13DF5D96F6DF98053CC
removing stale semaphore last operated on by process 11513 with name 0eBlobRegistrySignal_A8F5D998AC40B13DF5D96F6DF98053CC
removing stale semaphore last operated on by process 11513 with name 0emSteamEngineInstance
removing stale semaphore last operated on by process 11513 with name 0eSteamEngineLock
Installing breakpad exception handler for appid(steam)/version(1355957371_client)
The program 'steam' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadRequest (invalid request code or no such operation)'.
(Details: serial 33 error_code 1 request_code 153 minor_code 19)
(Note to programmers: normally, X errors are reported asynchronously;
that is, you will receive the error a while after causing it.
To debug your program, run it with the --sync command line
option to change this behavior. You can then get a meaningful
backtrace from your debugger if you break on the gdk_x_error() function.)
threadtools.cpp (2888) : Assertion Failed: Illegal termination of worker thread 'UICEFThread'
Assert( Assertion Failed: Illegal termination of worker thread 'UICEFThread' ):/home/buildbot/buildslave_steam/steam_rel_client_ubuntu12_linux/build/src/tier0/threadtools.cpp:2888

Installing breakpad exception handler for appid(steam)/version(1355957371_client)
Uploading dump (in-process) [proxy '']
/tmp/dumps/assert_20121220180832_1.dmp
success = yes
response: CrashID=bp-58c8a85f-88db-49c2-b3ff-32b512121220

System info:
Linux transition 3.2.0-35-generic #55-Ubuntu SMP Wed Dec 5 17:42:16 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
Intel corei7, 32G ram
RADEON HD 4670
Ubuntu 12.04 up to date