I've got 3 friends who have not played ME1 but played through ME2 and they loved it. Simply starting ME2 you can still get a default "this is the choices your character had made" style of game... really no different than any other game. Of my 3 friends who have played ME2, but not ME1, each of them have beat the game and two of them are trying ME1.... the one who is not trying ME1 is readdicted to Dragon Age (yay go bioware).
So, at least in my experiences here, ME1 is not at all hurting ME2. Sales are higher and word of mouth has been great. I played through it already and noticed no issues which would prevent me from playing the game. Hell, even the first one the only annoying parts were wandering eyes and loading elevators (great for loading bowls tho!). Anyways, I don't think that the sales have fallen like a "rocket," I think they are quite well :)







