Zombipanda
My Boom-Boom's mostly gay
Attacking Earth was the ultimate goal of the Collectors in the second game, though they never got that far. You can visit the moon and have Shepard be from Earth, but other then that, no.
But yeah, I'm also very excited.
Oh yeah! It's been a while since I've played it.
My best bet would be that, in the same way ME2 was about getting together a crew to strike at the Collectors and protect Earth, ME3 will be about building an alliance of the various races to protect Earth from a full assault.
But that's just a guess.