1) The only reasons I can think of for a 13" iPad are:
- a drawing tablet for artists (assuming they are ready to add a pen and digitiser)
- it would be the first iOS device that would actually sport a "full-sized" keyboard (the wireless hardware keyboard size).
2) I can't believe on the whole thread no one has made the obvious dumb joke ...
3) All the comments about making a more OS X like iPad, or a more iMac like iPad make no sense at all. It would be far easier to simply add a touch screen layer to the iMacs and MacBook Airs of the world, than it would be to shoehorn an iPad into being the laptop it was never meant to be or the desktop it was never meant to be.