I'll admit that the PR is worrying, I certainly don't want the entire focal point to be earth. However, I just can't see that happening, your going to have to travel around to god only knows where, otherwise the Normandy, Joker, EDI, all that stuff becomes almost meaningless if you stay on earth, and there has been way too much time invested in those things and characters by Bioware for that to happen. Space travel is a critical point of the series, I'll eat a small boulder if we aren't hopping all over the universe.
May I present an alternate theory as to what the trailer could suggest? London Jeff, Big Ben, or whatever you want to call him, talks about millions being dead, and that similar reports are coming in from around the world, if this is true, things are real bad already, and Shepard isn't there yet.
Consider the fact that Bioware likes to start it's games with major defeats. KOTOR, ME 1, ME 2, DA: Origins, all being recent examples of this. Combine this with the idea that trailers usually only show early to mid story concepts and not end game scenarios, throw in the actual information from the trailer, and I suggest to you that it is possible that Earth is lost sometime in the first third of ME 3, wiped out by the Reapers despite your efforts.
Sound unlikely? Maybe, but there is just as much evidence to support this claim as there is to support the whole game is taking place on, around, and about earth. So let's just take a deep breath, calm down, and wait until we have a little more information before worrying about it, either way.