This has been a known issue for a few years, but I guess the only reason the problem has not been fixed is because of the lack of users running nvidiabl.
Using nvidiabl to adjust screen brightness, when setting screen brightness down and running either a flash video online like youtube or a video in mplayer with vdpau causes the screen brightness to reset back to max brightness.
I personally use nvidiabl and experience this problem, and there are a few reports on this;
The developer has stated this is a known issue inside libvdpau that he can’t fix.
We need some type of fix in vdpau from Nvidia PLEASE… :)