What happened on the system since the last time Steam worked?
Nothing except update && upgrade.
But it seems to be a problem with the windows version too.
https://www.google.fr/search?q=!"Fatal+Error%3A+VGUI_Setup+failed"&oq=!"Fatal+Error%3A+VGUI_Setup+failed"&aqs=chrome.0.57.899&sourceid=chrome&ie=UTF-8
This is a very generic error that means Steam was unable to start, so it will show up for many reasons. As you might have seen from the Windows posts people often reinstall Steam entirely, and you can try that. Other than that, trying to narrow down what has changed since Steam worked last is the best way to try and pinpoint what is causing the problem.
I will try that so.
Regards
Closing pending more information that there's a Steam issue.
Nothing extracted yet.
It worked before ::
~$ steam
Installing breakpad exception handler for appid(steam)/version(1358447263_client)
Installing breakpad exception handler for appid(steam)/version(1358447263_client)
unlinked 0 orphaned pipes
Main.cpp (1824) : Assertion Failed: !"Fatal Error: VGUI_Setup failed"
Assert( Assertion Failed: !"Fatal Error: VGUI_Setup failed" ):/home/buildbot/buildslave_steam/steam_rel_client_ubuntu12_linux/build/src/steamui/Main.cpp:1824
Installing breakpad exception handler for appid(steam)/version(1358447263_client)
Uploading dump (in-process) [proxy '']
/tmp/dumps/assert_20130123004522_1.dmp
success = yes
response: CrashID=bp-72abd64a-3b58-4043-8c98-058bb2130122
Shutting down. . .
unlinked 0 orphaned pipes
CAsyncIOManager: 0 threads terminating. 0 reads, 0 writes, 0 deferrals.
CAsyncIOManager: 119 single object sleeps, 13 multi object sleeps
CAsyncIOManager: 0 single object alertable sleeps, 1 multi object alertable sleeps