I think this is turning more and more into a 2 horse race. The others, maybe with the exception of Microsoft, don't have a prayer to make a dent in the market. And even Microsoft may be coming too late to the party.
I think it will be very difficult to dislodge both Android and Apple from the top 2 positions. For one, Android has the advantage of being open source, and it is found in more and more markets (phones, tablets, eReaders, cars, GPS, and more to come, like house appliances), whereas Apple has the brand name, though I think it's starting to lose its shine.
Although the tablet market is still in it's infancy, I think it's too late for Microsoft, they may carve a niche for themselves in the corporate world (maybe taking RIM's market share), but I don't see them making it big in the consumer market. For other areas (eReaders, GPS, ...), Microsoft holds no advantages over Android.