Fog is being incorrectly rendered with accelerated X.
I've seen the same problem even with TNT2.
To test the problem you could use this source file:
You can try to hit key 'r'/'R' to scale up/down the size of red triangles - the fog is wrongly applied - I think
the latest Mesa3.2.1 includes fix for software rendering.
The same problem is with UtahGLX