I don't know if this is a bug in steam or gnome but i thought i'd report it both places.

Here is my post from steam for linux github (https://github.com/ValveSoftware/steam-for-linux/issues/2113):

Update
Seems to be an issue related to gnome. Cannot reproduce this in KDE and DWM or when i change TTY and start it in a seperate x server: "xinit /usr/bin/steam -bigpicture -- :1".

Original post
I'm also seeing this regression. I tested this both with and without STEAM_RUNTIME=0, no change. Is there any flag that forces steam to use the shared sdl2 library? (it would probably break as it's the latest mercurial checkout, but i'd like to test it).

terminal output
Running Steam on arch  64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
unlinked 0 orphaned pipes
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
[0310/003631:WARNING:proxy_service.cc(646)] PAC support disabled because there is no system implementation
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
Installing breakpad exception handler for appid(steam)/version(1361807486_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(1361807486_client)
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
Adding license for package .... #cut multiples
roaming config store loaded successfully - 3684 bytes.
migrating temporary roaming config store
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
ExecCommandLine: "~/.local/share/Steam/ubuntu12_32/steam"
Generating new string page texture 71: 128x256, total string texture memory is 442,37 KB
Generating new string page texture 72: 128x256, total string texture memory is 131,07 KB
Generating new string page texture 73: 64x256, total string texture memory is 507,90 KB
Generating new string page texture 74: 32x256, total string texture memory is 540,67 KB

(steam:30775): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent. #cut multiples
System startup time: 7,66 seconds
Running Steam on arch  64-bit
STEAM_RUNTIME has been set by the user to: ~/.local/share/Steam/ubuntu12_32/steam-runtime
ExecCommandLine: "~/.steam/root/ubuntu12_32/steam steam://open/driverhelperready"
ExecSteamURL: "steam://open/driverhelperready"
Installing breakpad exception handler for appid(steam)/version(1361807486_client)
Uploading dump (in-process) [proxy '']
/tmp/dumps/crash_20130310003648_1.dmp
success = yes
response:  CrashID=bp-693869e2-f1d6-48c8-a76b-fd86f2130309
~/.local/share/Steam/steam.sh: line 700: 30775 Segmentation fault      (core dumped) $STEAM_DEBUGGER
"$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
gdb backtrace
Replaced my home directory with "~" in case someone wants to diff.

ExecCommandLine: "~/.local/share/Steam/ubuntu12_32/steam steam://open/bigpicture"
ExecSteamURL: "steam://open/bigpicture"
[New Thread 0xe0d4bb40 (LWP 1266)]

received signal SIGSEGV, Segmentation fault.
0xf7eeab55 in XCreateIC () from ~/.local/share/Steam/ubuntu12_32/steam-runtime/i386/usr/lib/i386-linux-gnu/libX11.so.6
(gdb) bt

+ Trace 231655

    
Network Information:
    Network Speed:  
    
Operating System Version:
    Linux (64 bit)
    Kernel Name:  Linux
    Kernel Version:  3.7.10-1-ARCH
    X Server Vendor:  The X.Org Foundation
    X Server Release:  11303000
    X Window Manager:  GNOME Shell
    Steam Runtime Version:  steam-runtime-release-i386_2013-02-22
    
Video Card:
    Driver:  NVIDIA Corporation GeForce GTX 560 Ti/PCIe/SSE2

    Driver Version:  4.3.0 NVIDIA 313.26
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 60 Hz
    VendorID:  0x10de
    DeviceID:  0x1200
    Number of Monitors:  1
    Number of Logical Video Cards:  1
    Primary Display Resolution:  1920 x 1080
    Desktop Resolution: 1920 x 1080
    Primary Display Size: 18,78" x 10,55"  (21,54" diag)
                                            47,7cm x 26,8cm  (54,7cm diag)
    Primary Bus: PCI Express 16x
    Primary VRAM: 1024 MB
    Supported MSAA Modes:  2x 4x 8x 16x 
    
Sound card:
    Audio device: Realtek ALC889
    
Memory:
    RAM:  16037 Mb
    
Miscellaneous:
    UI Language:  English
    LANG:  en_US.utf8
    Microphone:  Not set
    Total Hard Disk Space Available:  80323 Mb
    Largest Free Hard Disk Block:  44188 Mb
    
Installed software:
    
Recent Failure Reports:
    Sat Mar  9 18:58:20 2013 GMT: file ''/tmp/dumps/crash_20130309195817_1.dmp'', upload yes: ''CrashID=bp-7ff57f97-03bd-4ff3-ad85-f822d2130309''
    Sat Mar  9 19:38:15 2013 GMT: file ''/tmp/dumps/crash_20130309203811_1.dmp'', upload yes: ''CrashID=bp-6757d6e1-acca-4b2e-891f-733a92130309''
    Sat Mar  9 21:07:18 2013 GMT: file ''/tmp/dumps/crash_20130309220714_1.dmp'', upload yes: ''CrashID=bp-bef65d5f-779c-41f0-bc49-678ee2130309''
    Sat Mar  9 21:19:56 2013 GMT: file ''/tmp/dumps/crash_20130309221951_1.dmp'', upload yes: ''CrashID=bp-50992572-bc88-45c3-9f40-c8c3e2130309''
    Sat Mar  9 21:42:11 2013 GMT: file ''/tmp/dumps/crash_20130309224207_1.dmp'', upload yes: ''CrashID=bp-f5138196-1da1-44cf-8bd1-82e442130309''
    Sat Mar  9 22:14:06 2013 GMT: file ''/tmp/dumps/crash_20130309231402_1.dmp'', upload yes: ''CrashID=bp-0b5d4e61-f956-44b4-8960-d8be92130309''