I think it actually is intended to be compatable with built in receivers by reading this on M$:

http://support.microsoft.com/kb/883970/en-us

Q4: I have a computer that has Bluetooth built in. Can I use the Microsoft Bluetooth-enabled keyboard and mouse without using the USB transceiver?

A4: You might have this question if you have a portable computer that is Bluetooth-enabled. The Wireless Optical Desktop for Bluetooth includes the USB transceiver because few desktop computers support Bluetooth. If your computer has Bluetooth built in and if your computer has the Human Interface Device profile, you may be able to connect the Bluetooth-enabled keyboard and mouse directly. If you want to use a specific internal Bluetooth-enabled radio, contact the manufacturer of your portable computer to determine whether the internal Bluetooth-enabled radio is supported by Windows XP Service Pack 2. Over time, this issue may disappear as manufacturers offer newer radio firmware.
So i'm not giving up all hope...thinking there has got to be something i'm missing here... maybe some booze and a big hammer will fix it...