Update: Microsoft sent us another statement, this time ruling out the date entirely. "An incorrect release date for Mass Effect was inadvertently posted on Xbox.com Australia and has since been removed. We will keep you posted on the ongoing development of Mass Effect, however, we have no announcements regarding the game's release date at this time."
According to the Australian section of the official Xbox website, we may be finding ourselves incinerating aliens and threatening suspicious bartenders before the year reaches its midpoint. A product blurb dedicated to BioWare's galaxy-spanning sci-fi RPG, Mass Effect, currently lists a release date of 18 May 2007, a bit more specific than the "2007" placeholder appended to the likes of Forza Motorsport 2 (which is set to arrive sometime in May).
It's worth noting that Microsoft has yet to announce an official release date.
At the time of writing, they had not yet responded to our request for comment.