Ars Technica has an article on five different multitouch mouse prototypes from Microsoft Research. This video shows the mice, as well as some simple applications that make use of the multitouch input:
These prototypes seem to forego a physical button in favor of pure multitouch interaction. I believe a more likely scenario would be a mouse that has both a physical button, as well as an area that accepts multitouch interaction (perhaps the button itself would also act as a multitouch sensor). This would allow people to keep interacting with the mouse the way they are used to, but to also make use of multitouch interaction when they want to.
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