The 2.5x tree is the development tree. The kernel has changed a lot. In the past it was very similar to 2.4.x but internally a lot of stuff changed. Using a lot of patches it might be possible to run the module again but it is not recommended to use 2.5.x. For normal users it is useless. Only developers should use the dev kernels. Major changes (new drivers ..) most of the time get backports. Also the new kernel won't include that much changes for normal desktop users but more for servers to let linux scale better.
There are some old patches here you can try: http://thuis.zwanebloem.nl/nvidia.
And again don't use it if you aren't a developer. (developers can create the needed patch themself ..)