How do I revive the Magic Mouse after replacing batteries?
A simple click should do it. I find that it takes a second sometimes to reconnect so I usually click and move the mouse side to side until I see the mouse cursor move.

Here is the official document from Apple:


  If you turn your mouse or trackpad off
  or turn off the Bluetooth option in
  your computer, the connection between
  the computer and the mouse or trackpad
  will be lost. When you turn on the
  mouse or trackpad and Bluetooth, you
  can re-establish this connection by
  simply clicking the mouse button or
  trackpad surface.


http://support.apple.com/kb/HT3081