I think they have started production on TF2 and that it won't be out until 2009. My brother has been a huge fan of TF since he was very little and he was showing me a site with some spoliers for the next movie. I don't know how reliable any of the so-called "leaked" information is but some of the characters in the next movie will include:
Starscream (and probably many of the other Decepticon "jets"). From the spoiler I read it lead me to speculate on perhaps the Combaticons as well. Supposedly Starscream goes rogue in the next film and leads his own group of Decepticons.
Megatron will be revived by Soundwave
Long Haul, Scrapper, Bonecrusher, Hook, Mixmaster, Scavenger- The Constructicons who merge together to form the huge Decepticon robot Devastator.
Ultra Magnus- He's similar to Prime but transforms into a car hauler.
There was a rumor of the Dinobots.
I can't remember who else was listed.