It can only point to something related to your opengl then, did you update drivers, or change anything in your system?
GLSL is used by 1000s of people daily, no reports came in that it suddenly stopped.

It's also very strange that 2.49 release still works, did you load this one from blender.org, and tested recently?

>>> It can only point to something related to your opengl then, did you update drivers, or change anything in your system?
No...I don't think so...Or unintentionally.

>>> GLSL is used by 1000s of people daily, no reports came in that it suddenly stopped.
I didn't say it's stopped. It doesn't work since 2.5 Alpha and before and I think that the problem comes from a combination of Blender and my computer.
But if it work with 2.49 (that I still use nowadays to test some blends) and r32504, that's because GLSL can work...
The r32504 looks different from others : there is others executable files and the other files are not exactly the same at the same position.

I think this issue come as much from me as from Blender, but it must be a solution. And I don't know why there is this problem on my computer.

Maybe it's not so much the specific revision but the build, did you try the official 2.56 release?

Yes, I tried all official releases and a lot of unofficial revisions, but I only found the r32504 that works.
It can exist other unofficial revisions that works, but I didn't test them. I don't test all revisions from Graphicall or somewhere else.

It's a real mystery for me... we need tangible information which we can redo ourselves.
I only know that driver upgrading often helps. It's also useful to test on a different system to see what's configured there.

(moving to opengl tracker)

Moved from Blender 2.5 Bug Tracker to OpenGL errors

I see that I forgot to say that Blender crashes when I try to enable GLSL, but I think it's obvious.