FYI -- possible virus
Since last night, my antivirus software has been warning me every time I load a page with an embedded YouTube video in it that the page is infected with the "Actns/Swif.T" virus. Oddly enough, the YouTube site itself doesn't seem affected -- or at least doesn't trigger the warning -- only pages that contain embedded YouTube videos.

Googling around suggests that a number of other people are hitting this today as well, but I haven't seen any official announcements about it yet. Some reports suggest that if you play the affected video it redirects you to a site that tries to get you to click on a link that installs a nasty trojan.

It's always possible that this is a false positive and there's nothing to worry about; and if it is something to worry about it will likely be fixed soon enough. But just in case, you might want to be a little cautious hitting embedded videos for the next day or two.

ETA: Seems to be a false positive in CA antivirus; they've released an updated signature and I'm no longer getting warnings.