Bullshit, Youtube works very fine in the Steam overlay browser while playing TF2 (via key F4).
And by the way am I not going to extent my donator status if this problem forces me to switch to Windows 7 or something, it's simply not necessary that there is a flash movie ad in a player that isn't properly written for donators. If I wouldn't donate I would say it's "my problem", but not as a donator.
It's pure luck if it crashes the game (and often even Windows), and it's very annoying to restart the whole shit even on a i7 machine and a 17 Mbit/s downstream line.