I am running debian -currently testing/unstable mix but previously 3.0r1. I use the nvidia installer but have compiled my own kernel using the old fashioned non debian way as I have not had the time to figure out the debian way.
The error you mentioned only happened for me when the gcc version (current) was different to the one the kernel was compiled with. The base gcc for 3.0 is 2.95 but 3.3 is available in testing. If you run gcc --version it will tell you the version of gcc currently running. If there is a problem you could recompile the kernel with the current gcc version and then the nvidia installer should work fine.