256.44 cpu / X spike
Just in case there's something useful in the bug report, I noticed that with 256.44 I got a consistent/persistent /usr/bin/X cpu spike. About 50-95% cpu via htop. Dropping back to 256.35 seems to have resolved the issue.
I watched htop for a while to make sure it wasn't something else, but I'm not completely sold that it was an nvidia bug, could be something else, but here's the bug report in case you see something in it.
The system is an odd Debian Sid but with kde 3.5.9, usually no issues with stable nvidia drivers, but certainly not an ideal test specimen for bug reports.