Microsoft insists tablets are Windows-powered mobile PCs. Even Windows Phone president Andy Lee, who should be pushing for a WP7 tablet, said, "We view a tablet as a PC."
Since the early days of the Origami project, Microsoft has looked at the tablet as a desktop replacement. Slates ran Windows XP or Windows 7; never Windows Mobile or Windows Phone 7. This desktop approach hasn't worked so well in the past, but the company just won't let it go.