Would you honestly be surprised if, with the announcement of the shipping date there are also new features?
I'm with you. It is so obvious that this device should have a front facing camera for video iChatting. Apple knew that and so planned on including it as evidenced by the space in the case. Cost issue? I think the component cost it a few dollars. Assembly cost is probably unmeasurable small given the automated assembly lines. That leaves either integration (software/firmware/battery/?) or marketing. The have two months to work out tech issues, and if they can't, they launch as demo'ed.
Or it could be marketing: leave it out. create hubbub. know that space in frame will be discovered. know that SDK will be dissected. gauge reaction to price as demo'ed, find most people view it as acceptable. keep price but add camera at launch and reap benevolence points.