hmm.. this happened to me when I went from 2.6.0-test4 to 2.6.0-test5 - exactly the same message. I had just used the same nvidia.ko I'd made for test4 and forgot to regenerate it with the 2.6.0-test5 kernel headers.. I just recompiled it using the instructions from minion.de to and copied it across and depmod'd ... and then it worked fine. Same thing applied to nvnet also. I did not change anything in the XFree config.
I have test4 and test5 in parallel on my SN41G2.