Here are two images of Sentinel Prime and an upgraded Optimus Prime for Transformers: Explosions and Noises 3. Or something like that.
In the comics Sentinel Prime was the Prime before Optimus and was defeated by Megatron (depending on which version you read) before handing the Matrix of leadership onto a young and unwilling Orion Pax, who then became Optimus Prime the new Autobot leader.
Michael Bay has said that in Dark of the Moon, Sentinel will be a "big brother and mentor" to Optimus Prime. Other trailer hints allude to him being buried in or on the moon. But no clue as to whether it will be a tale of mentor vs student if Sentinel turns bad.
Bay promises this one will be more "grown up". So who the fuck is he going to get to direct it for him then?