GameStop in the USA are already selling codes for some Mass Effect 3 DLC (called From Dust, so yeah the picture is related) despite the fact that the game isn’t even out yet. The codes are $10 so it’s around the usual 800 Microsoft Points/£7.99. Whether or not it will be sold outside of the US is yet to be determined, but I sure hope so.
Despite the fact that they are already selling codes, nothing is really known about the DLC. It might be the unknown and mysterious new squad member and extra mission that you get in the Collectors Edition (as Bioware have stated that these will be available from the release date), or it might be something else. Bioware and EA have yet to announce any details. I’ll keep an eye on it in case this situation changes.