According to 8BitJoystick.com, Microsoft definitely has a Wiimote-clone in development. The project has the internal Microsoft codename of "Newton" and could be a few months away from prototype.
The interesting thing is that Nintendo does not own all of the patents for the Wiimote's technology. A few of them are owned by a company called Gyration and Microsoft could conceivably license the tech for their own use. Nintendo hired Gyration to help them develop the Wii remote and licenses the tech from them.
We could definitely see Microsoft trying to do something like this as they continue to chase a broader market. This would also be something that current Xbox owners could buy without having to get a new console.
Now, if only they would port Smash Bros. to the 360.
Remember, this is all rumor at this time.
8BitJoystick: Xbox 360 "Newton" Motion Sensing Controller Confirmed