In a move to prepare users for upcoming OS X Lion multitouch gestures, Apple has made the Magic Trackpad a free build-to-order option in lieu of a Magic Mouse when configuring a new 2011 iMac. However you can still get both the mouse and the trackpad for $69.
Lion promises to bring more of what we have come to love about multitouch gestures in iOS and the trackpad will take full advantage of this. It would also be great for use with the magnetic iPad stylus for drawing apps such as SketchBook Pro or Photoshop.