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Most of the speculation is about a tablet, and I think this is a reasonable educated guess. At E3 we saw "Smart Glass", which allows tablet users to interact with the X360, which has become much more than a simple game playing device. It connects the user in new ways, so that the tablet can not only be used to interact with games, a la Wii U, it can interact with HBO Go, allowing new levels of interconnectivity and immersion.
Also, I've had the opportunity to play around with the Nokia Lumina 900 Windows Phone, and I think the UI would be fantastic on tablets. I say this from the perspective of a dedicated Android user! So there are lots of pieces in place for MS to make tablets that offer a compelling reason to buy them. If they can do it at a reasonable price and still offer solid hardware, I could see them being able to mount a real challenge to Apple's dominance in the tablet market.
Of course, last week they added big on-screen ads to skype, which makes me never want to use it again, so MS could screw it up just as easily.