OK, so I've found out that the power source does
get updated, but not for perhaps 10-30 minutes. I've attached an nvidia-bug-report, but it's too big when compressed with gzip or bzip2 to be accepted, so it's lzma compressed (rename, then unlzma should restore it to a text file).
More information on this problem: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=629418
Someone else with the same symptoms, albeit a rather older driver version (185): http://www.nvnews.net/vbulletin/showthread.php?t=133555