I am having the exact same problem with my Magic Trackpad, and it is only a few months old. Installing new batteries did not help. If I restart my Mac Mini the Trackpad connects. It will work for a while and then randomly lose connection.
When it disconnects, if I go to the System Preferences Pane and click on the Trackpad icon nothing happens. Also, if it is disconnected and I click on the Bluetooth icon in System Preferences I get the spinning beach ball and either have to force quit it or restart the computer. Both icons work fine when the Trackpad is connected.
I have no other Bluetooth peripherals paired with my Mac Mini and I have not made any changes to the system recently.
Any suggestions would be appreciated. Thanks.