geek.soosoo.at

sudo mkdir /etc/adobe
sudo su
sudo echo \"OverrideGPUValidation = 1\" >> /etc/adobe/mms.cfg

works for me.

Reference:

http://ubuntuforums.org/showthread.php?t=1593363&page=4
http://www.noobrescue.com/blog/flash-fullscreen-doesnt-work-on-ubuntu-10-10