Here's how you know an official Android tablet is being worked on in some back room somewhere. When speaking with Google VP Andy Rubin earlier today, the subject of an Android tablet was brought up. When asked wither or not we'll be seeing one before long, Rubin was ready with the stock response of, "We haven't announced anything." Running that through my handy PR-gibberish translator I come up with "You are barking up the right tree, but today's not the day for that."
Interestingly enough, this was essentially the same answer he gave last month at Google I/O when pressed about a "Nexus Two". Oh, and who could forget the classic "We (Google) don't make hardware" which prefaced the eventual release of the Nexus One?