Comment 23 for bug 146710

It seems that as of now, the only option is to deactivate the nvidia driver (temporarily via switching to console mode).
This bug is known to NVIDIA for one year now at the very least, as can be seen in

http://www.nvnews.net/vbulletin/showthread.php?t=100494

Yesterday there was a post of someone who succeded in changing the brightness under X via a hardware hack:
http://www.nvnews.net/vbulletin/showpost.php?p=1815030&postcount=133