What is Facebook about to unveil?

As we previously reported, Facebook is holding a special mobile event at its headquarters tomorrow where it will announce ...  something.

Will this be the announcement of the previously denied "Facebook Phone"?  Or is Facebook announcing new mobile applications for other operating systems, like Android and iOS.  That former Android lead Erick Tseng moved to Facbeook in May is a great source of speculation, there's really no evidence out there.  They've kept a pretty tight lid on what the announcement will entail, so your guess is as good as ours.  If you have a prediction, share it with us in a comment below.