I have a theory as to why the iPad does not have a built in camera. I think Apple will announce a camera accessory that plugs into the dock connector and swivels 180 degrees to point forward or backward. There is no "up" on the iPad -- all orientations are required to be supported by applications -- so it does not matter that the dock connector is on the "bottom" of the device. In the front-facing position, the camera would enable video chat. Turn it around and you can take pictures with the gazillion iPhone apps with that function built in. This solves the two camera problem, gives people the camera functions they want, and gives Apple another high-markup item to sell.
P.S. Sorry for the slew of iPad posts, but I think it's a pretty cool device.