Read this thread at your own risk as it will contain a lot of spoilers!
A person working on the third Transformers movie has taken upon himself to supply the fans with spoilers and info about the movie. Although he hasn't revealed himself or what his job on the movie is, many of the things he's revealed have been shown to be accurate afterwards. He posts on a few different forums so I thought I'd try to gather some of the stuff in a thread for you guys here. Please remember that more or less everything is unconfirmed, and things will change in the movie right up until release.
Again, you read this at your own risk!
* Optimus has a trailer (previously confirmed). The trailer is part of a weapons system
* Lots of new Autobots and Decepticons, and this time we get more new Autobots than Decepticons
* Still not sure if the twins are in the movie even though several ice cream trucks have been seen on set. They could be part of a flashback scene that's included in the movie. The truck looks brand new and shiny as well, compared to the rusty, crappy one that was in ROTF
* Simmons is back, and he's a millionaire, possibly from writing books. Less toilet humour as well
* Megatron is back, but with a twist. Apparently he's back in a way that "no one expects"
* The red Ferrari is Hot Rod
* The red firetruck is Inferno
* The reason for fewer new Decpeticons is because the Autobots are fighting more than Decepticons this time, and it is because of something the 'cons did
* Patrick Dempsey might be a pretender
* Rejected movie titles include "Exile", and "Key to the Past"
I will keep adding to this post as new stuff gets released!
UPDATE:
* story will return to core mythology with more character development and story telling
* The main bad ground will be Shockwave in a story that goes "into the space race between the U.S.S.R. and the USA, suggesting there was a hidden Transformers role in it all that remains one of the planet's most dangerous secrets. 'The movie is more of a mystery,' Bay says. 'It ties in what we know as history growing up as kids with what really happened.'
* The meta plot is "Sam Witwicky (LaBeouf) taking his first tenuous steps into adulthood while remaining a reluctant human ally of Optimus Prime."
* Patrick Dempsey's role is described as "significant to the plot"
* Starscream is killed by a human, "Hardcore Eddie"
* Story includes the possible assassination of humans by two robots
* "As a trilogy, it really ends," [Michael Bay] says. "It could be rebooted again, but I think it has a really killer ending."
* Jolt the Chevy Volt does not return for TF3