If you’ve a) recently upgraded to Lion; and b) are shouting this a lot every time you wake your Macbook Pro up from sleep, I’ve got a fix for you.
The symptoms are that once you’ve woken the Macbook up, your Magic Mouse has been disconnected. Clicking the ‘connect’ option from the Bluetooth menu apparently connects to the mouse, but nothing happens. The only way to get it to work again is to open the mouse preferences pane, at which point it starts to work again.
I’ve no idea WHY this was happening - there’s nothing in the log files to suggest a problem, but I have got a fix. Power down the machine, then reset the System Management Controller. That’s done by holding down the left-hand shift, control and option keys while powering on. Release the shift/control/option keys as the machine boots back up again.
That should nail the problem. Normal post-wakeup mouse service will be resumed.