While Mass Effect 3 can already be pre-ordered, don't discount your crew from part 2 just yet.  Arrival is the last piece of DLC for Mass Effect 2 and allows some more gameplay to help tide you over until the new game release slated for sometime later this year.

For plot details of the DLC check out Bioware's site.  The cost is 560 Microsoft Points for 360 gamers, 560 Bioware points for PC users, and $6.99 for the PS3.