You guys are going to far in assuming that Cerberus is correct in believing Shepard dead. Not only have all been advised on the official forums to keep their saves (and quote from 'Evil' Chris Priestly: "The trailer tells you nothing") but they have not altered their original stance that Shepard from 1 carries through
Rampant speculation time: Several remarks made in a documentary on the platinum hits version had to do with the game exploring the nature of being and in particular the humanity/value of someone or something like an AI. The trailer shows what appears to be the N7 (Onyx)armor ON something Geth. Remember also that the Geth are leaderless after Sovereign (their GOD) is destroyed BY SHEPARD. They are also capable of thinking independently and were building shrines in Mass Effect 1.
When you consider that Bioware has a penchant for people thinking significant characters are dead (look no further than Revan, or heck, Shepard for a few minutes already), for following Campbellian Hero's Journey plots, and have even had the player character survive death and come back according to the "Journey to hell" part of the motif in Jade Empire, I start to think we'll be seeing cyber-shepard of some kind trying to regain humanity with some flavor of sympathetic splinter Geth. Speculation, sure, but not baseless at all.