Although that teaser shot from Microsoft showed a flat contraption, the product will indeed feature an arc-like design... but only if users wish it so. WinFuture explains that the rear of the mouse can either be kept flat or "bent by the user as needed." Interesting.
Unfortunately, this story also dashes our hopes for a multi-touch device. The Arc Touch Mouse reportedly has an "advanced touch-scroll function" instead of a scroll wheel, but WinFuture says nothing about actual multi-touch functionality a la Apple's Magic Mouse. Also, the photos appear to show a pair of conventional-looking buttons. (Other rumored features include a BlueTrack optical sensor, 2.4GHz wireless connectivity, and a "nano-transceiver that can be stored in the battery compartment.")
Although Microsoft has yet to make an official announcement, the Arc Touch Mouse is already listed on Amazon Canada. The e-tailer charges $79.95 CAD, which ain't cheap for a non-gaming pointing device. Considering that Amazon sells the existing Arc Mouse for $45.99 CAD in Canada and $31.99 USD in the States, though, you can probably expect U.S. pricing to end up a ways below 80 bucks. (Thanks to Engadget for the tip.)