almost the same error over here, ati radeon mobile 7500, Arch Linux.
3d is enabled (using DRI), and it's the same xorg.conf as my ubuntu install, where everything worked fine.
Here's the log (not sure how different it is)
tremulous 1.1.0 linux-x86 Jun 14 2006
----- FS_Startup -----
Current search path:
/home/gimmick/.tremulous/base
/opt/tremulous/base/vms-1.1.0.pk3 (4 files)
/opt/tremulous/base/map-uncreation-1.1.0.pk3 (110 files)
/opt/tremulous/base/map-tremor-1.1.0.pk3 (45 files)
/opt/tremulous/base/map-transit-1.1.0.pk3 (135 files)
/opt/tremulous/base/map-niveus-1.1.0.pk3 (134 files)
/opt/tremulous/base/map-nexus6-1.1.0.pk3 (151 files)
/opt/tremulous/base/map-karith-1.1.0.pk3 (118 files)
/opt/tremulous/base/map-atcs-1.1.0.pk3 (87 files)
/opt/tremulous/base/map-arachnid2-1.1.0.pk3 (67 files)
/opt/tremulous/base/data-1.1.0.pk3 (1229 files)
/opt/tremulous/base
----------------------
2080 files in pk3 files
execing default.cfg
couldn't exec autogen.cfg
execing autoexec.cfg
execing quickChat.cfg
Hunk_Clear: reset the hunk ok
----- Client Initialization -----
----- Initializing Renderer ----
-------------------------------
----- Client Initialization Complete -----
----- R_Init -----
------- Input Initialization -------
Joystick is not active.
------------------------------------
...loading libGL.so.1:
Calling SDL_Init(SDL_INIT_VIDEO)...
SDL_Init(SDL_INIT_VIDEO) passed.
Initializing OpenGL display
...setting mode 3: 640 480
Using 8/8/8 Color bits, 24 depth, 8 stencil display.
GL_RENDERER: Mesa GLX Indirect
***********************************************************
You are using software Mesa (no hardware acceleration)!
Driver DLL used: libGL.so.1
If this is intentional, add
"+set r_allowSoftwareGL 1"
to the command line when starting the game.
***********************************************************
...WARNING: could not set the given mode (3)
----- CL_Shutdown -----
RE_Shutdown( 1 )
-----------------------
----- CL_Shutdown -----
-----------------------
Sys_Error: GLimp_Init() - could not load OpenGL subsystem