OMG.....

Sh-sh-sh-sheperd.....D-d-dead...WH...WHAT!?!?!?!?

How can this be?

Well, obviously this seems to be a huge and true 'teaser' trailer, because in the end it is really teasing all those who have played through Mass Effect and have become fans of the game.

My theory: (SPOILER ALERT FOR THE BOOK MASS EFFECT: ASCENSION):

Cerberus is going after Sheperd and in order to lower Sheperd's attraction to Cerberus, the Alliance has faked his death to get Cerberus off his tail and allow him to take clandestine operations against them...

Which if THAT is true, it could mean that Sheperd loses all his amor, weapons, and weapons upgrades (but not his stats, of course, that would go against what Bioware said about allowing players to use their existing characters from the first game for the next two, then again, this could be a sign that Bioware was not able to pull this off, which would be hugely dissapointing). Sheperd would have a new identity, which would help give explanation of starting the game with low-level guns (makes sense, wouldn't starting Mass Effect 2 with a level 60 character and ALL their high-level upgrades, weapons, and armor make the game a tad to easy?).

OR....Cerberus captured Sheperd somehow, and the party members are going to have to find him (which hints at the possibility of 4-player co-op, the scene where the trailer shows Sheperd's "Alien 4 Associates" might be a clue).

But I would go with my first assumption, seems more realistic, and it would not piss me off like the second option would.