Microsoft has a history of playing catch-up in the marketplace. Because of their need to "get it right" rather than "get it fast" (for a variety of business, technical, and legal reasons which I won't expound on here), they have often shipped bad products when the opportunity existed and then later shipped good versions of the products but after the window of opportunity had closed. Some of the highlights (lowlights?) of this process are: Windows Mobile, TabletPC, Zune, and Bing.
According to this article, Microsoft could break that cycle right now:
By extending this core competency to a new Windows Phone 7-based tablet operating system, Microsoft could offer users the best of both worlds — a welcome alternative to both Google's "fragmented" and Apple's "closed" approaches.
Only time will tell, of course, and I don't expect to see a Microsoft tablet in the next six months. But if one appears, well, it could get interesting...