There's only one episode up as of this writing, however, and a little birdie tells me that the episode is "b0rked" in technical terms-- apparently the quality isn't very good, and it may have simply been a test to start getting the show online. Unfortunately, while the episode posted is pretty good (it stars Drew Berrymore as Brian's dumb girlfriend), we have no idea what else is going to show up in iTunes-- if they're going to do full seasons, or a "greatest hits" of downloads.
Either way, Family Guy is on iTunes. Welcome, Griffins.
Thanks to everyone who sent this in!