According to Eurogamer, Microsoft calls Iris a "spiral campaign" (clever!), promising to reveal details about the Forerunners, as well as other elements of the heady Halo mythos.
GamesIndustry.biz reports that the campaign officially began this past weekend, with the distribution of flyers in New York City and London, bearing the apparent logo of the campaign, and the message "We are not alone." At the moment, it's unknown who's behind the campaign, although the absence of the term "Alternate Reality Game" might indicate that it's not 42 Entertainment, the viral veterans behind Halo 2's ilovebees campaign.
Either way, Halo and conspiracy fanatics can get their non-game on by checking out the Society of the Ancients website... and this nifty countdown clock! Because we haven't had enough of those already, right?
Update: Let's not forget this cryptic e-mail from Microsoft.
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