We are live at the Transformers Prime panel at New York Comic Con 2011 where they revealed that Nathan Fillion (Firefly, Castle) will be voicing a Transformer in Season 2.

The creators of the show did not reveal if he will be an Autobot or Decepticon, but they did tease that anything can happen.

Mr. Fillion is just one of the celebrity voice actors that they wanted to work with and said to expect more guest starts just like this. [SOURCE]