They show Megatron transformed into a tank-like thingie resembling a hybrid of Batman's Tumbler, a TRON tank, and a Terminator HK rolly-aroundie thingie.
Personally, I think this is a vast improvement over the "alien jet" vehicle we saw in the first film. Provided, of course, this isn't part of the alleged "misinformation" campaign Mr. Bay claims is underway. Gottta be careful with that, though. Sometimes "misinformation" is cooler than what eventually ends up on screen.
Thanks to Colin for dropping us a line about this.