Take this with a pinch of salt, but a Joystiq reader has sent in a picture of a GameStop receipt for Mass Effect 3 DLC. Obviously there's no information about it, just the name, "From Dust", and the reader's story. He says he spent $10 on the pre-order, although there's no details on release date or anything.
So take this as news that Mass Effect 3 is likely to be getting DLC – hardly news when you think about it. This does raise an interesting idea in my mind though, something that I haven't really thought about this far before release (why would I? There's still loads of games coming out between now and March). How final can the end of Mass Effect 3 possibly be if there's going to be non-multiplayer DLC?
That's presuming that it's single player supplements, of course, but it's an interesting thought. Mass Effect 3 comes out March 6th.