Incidentally, whoever disabled the flash bbcode tag should whitelist youtube, since it's probably safe
There's no easy way to whitelist sites, flash is either on or off.
I turned it off due to the widespread exploitation of Flash vulnerabilities in the wild, coupled with the fact that some spambot posted a bunch of Flash videos on the site a few days ago. I figured that whole thing was an issue waiting to happen.
(Not to mention that people could use Flash to steal cookies and PM them to themselves.. Flash is just an accident waiting to happen)