Bluetooth Magic Trackpad not recognized after sleep

When I put my MacBook Pro (2011, 13") to sleep and wake it up again, the Magic Trackpad doesn't get recognized. I have to turn Bluetooth off and back on again to get the trackpad to work. It shows up in the Bluetooth menu at the top of the screen, but telling it to reconnect doesn't do the trick.

I have a USB keyboard, so I don't know if the same problem would be true of the keyboard.

Why might this be? Is there a way to have Bluetooth automatically restart when I wake the computer?

EDIT: I should mention that this has only happened since I upgraded from Snow Leopard to Lion last week.

Try re-pairing the trackpad. Go to System Preferences then Trackpad then Setup Bluetooth trackpad... Select the trackpad and then Unpair. Then select it again (using your built-in trackpad) and select Pair.