Of course, it’s IMDb, it’s all anonymous and uncheckable, but the Mad Men message board has this up, and there’s not actually a good reason to lie. The poster claims it is from an email from AMC. My source at AMC refuses to divulge, so either this is false, or it’s some AMC flunky who actually didn’t know any better than to leak, because I can’t imagine a controlled leak goes to an IMDb message board.

“Mad Men” will return for another season, beginning
Sun., July 27, 2008 at 10 pm (schedule subject to change). As with the
first season, Season 2 will include 13 original episodes.

And in case you missed it, we will feature a marathon of all of Season
1 episodes on Sun., July 20 (subject to change), beginning at 12 pm.
Every episode will air, back-to-back, through 1 am.