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On the iMac I use at work, I use Bootcamp to run Windows XP.  When the system wakes from sleep (suspend) the built-in Bluetooth device fails to wake up.  This is nearly 100% of the time.

Solution: I bought a little USB Bluetooth device, plugged it in, and disabled the built-in device.  After waking from sleep the external USB Bluetooth device ALSO fails to wake, but if I just physically disconnect it and then reconnect it, it comes to life and with a slight delay, my wireless keyboard comes back to life.