After installing the latest drivers.

http://support.amd.com/us/gpudownload/linux/legacy/Pages/legacy-radeon_linux.aspx

Steam will fail to load.

Installing breakpad exception handler for appid(steam)/version(1359221069_client)
SteamUpdateUI: An X Error occurred
X Error of failed request:  BadRequest (invalid request code or no such operation)
Major opcode of failed request:  139 (ATIFGLEXTENSION)
Minor opcode of failed request:  66
Serial number of failed request:  13
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:  136 (GLX)
Minor opcode of failed request:  19 (X_GLXQueryServerString)
Serial number of failed request:  29
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:  136 (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:  136 (GLX)
Minor opcode of failed request:  14 (X_GLXGetVisualConfigs)
Serial number of failed request:  51
xerror_handler: X failed, continuing
Looks like steam didn't shutdown cleanly, scheduling immediate update check
Installing breakpad exception handler for appid(steam)/version(1359221069_client)
Installing breakpad exception handler for appid(steam)/version(1359221069_client)
unlinked 0 orphaned pipes
removing stale semaphore last operated on by process 2184 with name 0eBlobRegistryMutex_5D734C8CACF793F0CB507AF201B5283D
removing stale semaphore last operated on by process 2184 with name 0eBlobRegistrySignal_5D734C8CACF793F0CB507AF201B5283D
removing stale semaphore last operated on by process 2184 with name 0emSteamEngineInstance
removing stale semaphore last operated on by process 2184 with name 0eSteamEngineLock
Installing breakpad exception handler for appid(steam)/version(1359221069_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 34 error_code 1 request_code 139 minor_code 66)
  (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 (2892) : 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:2892

Installing breakpad exception handler for appid(steam)/version(1359221069_client)
Uploading dump (in-process) [proxy '']
/tmp/dumps/assert_20130130214612_1.dmp
success = yes
response:  CrashID=bp-22d22c7c-4210-43ca-9312-393e62130130

I have tried reinstalling the .deb but I haven't uninstalled and reinstalled fully yet.