For one, co-op can only be played locally via Wi-Fi or Bluetooth. Additionally, it's only available in the game's "Viva La Turret" mode, with one player free flying and the other controlling the turret. (Also of note: finishing touches are still being put on the iPad version of the update.)
Of course, it's a freebie for anyone who's ponied up the buck for the Viva La Turret DLC, so we're not grousing too much. See what you're in for in the (very funny) co-op trailer after the break.
... So, umm, anyone wanna come over to our house and play? Mom made pizza rolls.