As to what the goodies will be, exactly, Bungie isn't saying -- "don't get too excited, it's not Halo 3," said the newsbrief. We're predicting gamertag pictures, themes, and the like, of either Halo, Marathon or their long-running joke Pimps at Sea.
Regardless of what the content is, there's only a small window of opportunity, as the mysterious content will only be available from 5:00 am ET until 3:00 am on Sunday.
[Via X3F]