Wikipedia has some pretty good info on the film and the Transformers showing up and returning, describing the premise for the film as thus: Sam Witwicky (Shia LaBeouf) discovers something about the origins of the Transformers and their history on Earth. The evil Decepticons need to capture him for information. The climactic battle takes place at the Giza pyramid complex, where a temple is located within. So apparently Megatron wasn’t the first Transformer to visit Earth and the temple in Egypt ties directly into those first visits thousands of years ago.
Some of the new Autobots this go-round include Arcee, one of the female Autobots, Jetfire (I always preferred the name Skyfire personally), who transforms into an SR-71, and Wheelie, whom I hope gets fragged very quick and very brutally because I hated him back in the animated feature. New Decepticons include the Fallen, Ravage, the one-eyed cat, and of course Devastator, whom I’m assuming is the big mo-fo monster-trucking everything.
One very interesting bit of fan wank mentioned by writer Roberto Orci was that, for the possible third movie, Unicron and some Triple Changers could be introduced. That’d be about the only way the threequel could be bigger than this thing, which is short on the words and big on the explosions. John Turturro, Josh Duhamel, Tyrese and Megan Fox are also set to return. Definitely looking forward to 6.24.09. Click below for the fireworks.