Microsoft IntelliMouse Explorer for Bluetooth Review @ The Tech Zone
 

There are two main difference between the new IntelliMouse for Bluetooth and the old IntelliMouse for Bluetooth. The first is the new tilt wheel that allows you to scroll web pages from side to side as well as up and down. The other major change is the Bluetooth receiver - the new unit is more than three time smaller than the old unit and plug directly into a USB port. Microsoft also tweaked the shape of the new IntelliMouse for even better ergonomics.

http://www.thetechzone.com/?m=show&id=130