Transformers 2 has a subtitle! In a posting made by producer Don Murphy on his forums, he linked to a blog post, revealing the full title to the sequel:

DreamWorks – Paramount Pictures have release the new title – Transformers 2: Revenge of the Fallen – for Transformer sequel directed by Michael Bay. The Office’s Rainn Wilson joins the original cast of Shia LaBeouf, Megan Fox, Josh Duhamel, John Turturro and Tyrese Gibson in Transformers 2: Revenge of the Fallen.

Transformers 2: Revenge of the Fallen is written by Ehren Kruger, Alex Kurtzman and Roberto Orci and produced by Tom De Santo, Don Murphy and Lorenzo Di Bonaventura.