Speculation from around the web:

The battle for Earth has ended but the battle for the universe has just begun. After returning to Cybertron, Starscream assumes command of the Decepticons, and has decided to return to Earth with force. The Autobots believing that peace was possible finds out that Megatron's dead body has been stolen from the US Military by Skorpinox and revives him using his own spark. Now Megatron is back seeking revenge and with Starscream and more Decepticon reinforcements on the way, the Autobots with reinforcements of their own, may have more to deal with then meets the eye.

Confirmed as in production, and set for a 2009 release...

I have to say, after the way the film ended its disappointing (but figured it would be the case) that the Military would have kept at least one of the dead bots. Of course they had to pick the leader didn't they!

Will they ever learn to just throw it all down to the bottom of the ocean...