No one knows yet. Most of the "choices" (such as they are) in ME2 you make will carry over (what squad members survived, etc), though you'll probably just get more email again like impact most of the "choices" you made in ME1 had.
I'm just hoping for for an ME1 to ME3 import tool...:laugh:
I finished the game with one character but i guess your choices in the previous games doesn't effect anything with the ending and I don't expect any major game changes from different playthroughs, maybe except for saving the council and wrex.
There's plenty of story-decisions that find that their way into ME3. I assume that the amount of credits, minerals & XP also get imported, in one way or another. (Starting creds, war effort, starting lvl?)