According to TFW2005, one of the main cast is gonna die in TF3.

Note that it is just a rumour at this stage.

In support of it being true - consider that this is supposed to be the climactic conclusion to Bay's 3-part story arc, expect carnage and quite a few deaths in this third one. Bay will want casualties!

Against this being true - Hasbro's four core characters for endless toys (and revenue) are Optimus, Megatron, Starscream and Bumblebee (all four were even inducted into the Hall of Fame this year)... so it is unlikely that Hasbro would allow for one to be permanently killed off, as it would impact heavily on their toy-character-product lineup. Sure, they've still been producing quite a few toys of characters that died in the first one, but imagine how many more we would have had if they didn't die, based on how many toys each of those four main characters have had.

Ultimately, it will be a test between who has more creative power/control over this movie franchise - Bay or Hasbro.