I havent found a way, flash simply aborts fullscreen when you remove focus.
I presume that they figure in a single screen scenario you can't click on anything else, but if you want to get rid of the screen, alt+tabing to another app will minimize it. Most other people rely on the Esc key for that. They probably think its a feature, but when you have dual monitors it blows (My second one is my TV)
"Hey Adobe! Maybe I DO want to watch a full screen flash video on one monitor while typing on the other screen thank you!"
Anyway, sorry I can't help, but I'll be watching in case someone knows of a flash reg hack that'll fix it or something...