Xbox.com will reach zero, thus signaling a time when Satan and His Minions will ride out from the Gates of the Underworld on flaming steeds of bone and hatred to conquer the world. Probably.
In the event his conquest gets delayed, we're wagering a guess that the countdown will coincide with a new announcement from Tim Schafer's Double Fine studios, who recently registered a trademark for the title Brutal Legend. Of course, with all the work put into Epic Saga: Extreme Fighter, we really don't know how the developer would have had time to work on two projects at once.
Double Fine's previous title, Psychonauts, was a critical darling whose lackluster sales can only be attributed to an unfortunately-timed pan-dimensional rift. Fortunately, we don't have long to wait and see what the countdown means (though its speculated connection to a Ninja Gaiden 2 announcement now appears unlikely).
Update: And Brutal Legend eludes us yet again. The countdown was for Ninja Gaiden 2 after all -- we're disappointed and overjoyed at the same time.