As noted on the NVidia Linux forums (http://www.nvnews.net/vbulletin/showthread.php?t=129711) there is a race condition in the XDamage extension that affects users of NVidia cards under compiz. A developer has added a commit to the workarounds plugin that resolves this (see URL) but too late to make 0.8.2. Adding this as a patch would make life easier for NVidia users as the issue can crash the NVidia driver entirely and lock up X. Please consider adding this.
Thanks for reporthing this. It's just been checked into the overlay, so please update it and re-emerge compiz-plugins-main. Feel free to reopen this bugs if you encounter any problems with the patch... 5:)