The reason 2.5.x kernels aren't support is very logic. The 2.5x tree is the development tree. Those kernels are only meant for kernel developers. In the past when 2.2x was stable (there were nvidia drivers already) and 2.3x was dev the drivers didn't support 2.3x either.
The 2.5x kernel is changing too fast. For normal users it is a very bad idea to use 2.5x at all. It can be a big risk. (experimental filesystem changes....)
You can try patches from here here: http://thuis.zwanebloem.nl/nvidia
I thought that patch doesn't work anymore because 2.5x changed a lot.
When 2.6x comes out nvidia will support it officially.