I have managed to solve this problem, here's what I did in case anyone else has the same problem:
My original nvidia source build .deb package (create with 'make-kpkg modules_image') was from the testing tree. I upgraded to the unstable tree (see http://home.comcast.net/~andrex/nvid...ian/index.html
for info on how to do this, it took a little work). Then I:
rm -rf /usr/src/modules
grabbed the make-kpkg .diff from the site mentioned above (make sure to grab the unstable .diff, as there is one for testing and one for unstable).
rebuilt the module from scratch according to instructions, and installed over the existing .deb created from the testing sources.
I believe the problem had to do with the TLS problem that I've read about, but completely don't understand (or care to).
Another note: even after I got into X, as soon as I ran glxgears, my system locked up completely. Switching the NvAGP option to "1" from "2" in my XF86Config solved this.
Hope this helps someone.