I've generally avoided allowing embedding of flash, and I'm still not sure about Youtube support. Flash had to do with being a bit of a security concern (holes in Flash is the source of many security vulnerabilities). I'm less concerned about Youtube from that perspective, but I'm not sure what the impact on the community would be.
I will think about it though. It's certainly becoming more and more common, but at the same time, some things that become "common" aren't always good for a healthy happy online community.
Thanks for the suggestion and link to the video on supporting Youtube via BBCode.