It seems reports of Leonard Nimoy's complete retirement from acting were a little premature, as he's going to be in Transformers: Dark Of The Earth. However Michael Bay doesn't have the octagenarian actor running from explosions, as EW reports Nimoy will instead voice Sentinel Prime, Optimus Prime's predecessor.
Sentinel Prime's wrecked body, which crash landed on the moon and was discovered by the Apollo 11 astronauts in 1969, was first seen in the teaser trailer for Transformers: Dark of the Moon. On earth, he will take the form of a fire engine, and serve in the sequel's climactic end battle sequence that takes place in downtown Chicago.
Michael Bay originally wanted to get Nimoy in Transformers: Revenge of the Fallen, but was afraid that he wouldn't be able to pay the actor enough money for the voice work. Bay says, "I was too scared to ask him. 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!'"
The reference to be being back refers to the fact he voiced Galvatron in 1986's animated Transformers: The Movie