Could he be appearing in Halo: Reach? Maybe it's something to do with a movie in the works? Whatever it is, "something big is dropping" soon.
Bungie got the rumour ball rolling when they "Just got this mysterious image and word that something big is dropping this week?!" Cautious fans have pointed out that tomorrow is April 1st, meaning it could be a Fools trap.:
Would Bungie be so cruel as to dangle a fake movie announcement, or the like, in front of salivating fans? Let us hope not. I guess we'll have to wait and see...