EW informs us that Leonard Nimoy will be voicing Sentinel Prime, the predecessor of Autobot leader, Optimus Prime.
Apparently, Sentintel Prime's body was found during the Apollo Moon Landing in 1969 and brought back to earth. EW states SP disguises himself as a fire-engine in the movies final battle.
Interesting to note, that Nimoy is no stranger to Transformers having voiced the mighty Galvatron in the original animated Transformers: The Movie. I suppose it also helps that Nimoy is married to Bay's cousin, as well!
“I was too scared to ask him,” said Michael Bay. “Plus, he’s married to Susan Bay, who’s a cousin of mine. So I had to be careful. I’ve met him at family functions. But he told me, ‘I would be honored. I’m glad to be back!’”
Transformers: Dark of the Moon hits theaters July 1st, 2011.
Here are some recent images released. Click to enlarge: