Well, 2.5 was mentioned because the phoebe 2.4.20 kernel has many things ported back from 2.5, the threading stuff + VM changes of which are relevant to this discussion. If they want to support RH 8.1 when it comes out, they'll have to make many of the modifications they would have to do to support 2.5.
The only real showstopper is the use of segment registers in libGL. That might be as easy as nvidia tweaking some compiler options and recompiling or it might be something that would require a rewrite of major portions of the code. Hard to say without seeing the code.
Using a non-RH kernel is always a solution, but then you lose the nice scalable threads etc.