I'm generally not a big fan of this play, as limp-calling with AA generally leads to many more headaches than you really want to have when playing a premium starting hand. If you raise upfront with it, you'll still get action from the KK-JJ, AK and the like. The few times I've tried to limp-reraise at a table that has been playing aggressively preflop, I almost always whiff. Looks like this one probably worked out for the best, though, unless he was making a move preflop with 99 or 77.