Oh yeah, the Microsoft Receiver allows connection of 7 devices. The mouse and keyboard count as 2, so that leaves you with 5 left. It does indeed work with other Bluetooth devices.
I purchased the MS Wireless Optical Desktop and returned it the next day. It was pretty crummy overall and the RF didn't reach over like 4 feet. I needed a setup for a HTPC that would allow me to work in a HT room that is 20x12 (built specifically for HT). The RF Optical desktop just couldn't do it. I purchased the MS Optical Desktop for Bluetooth. OMG it's the best wireless setup you can get. It allows for usage up to 30 feet (even through walls, RF can't touch that) and it's a SECURE connection (I won't say someone "can't" steal keystrokes from bluetooth, but anyone could with an RF device).
The mouse is the best optical on the market. Fresh Gear on TechTV gave it 4 outta 5 stars (lost one cause of price

) and beat out logitech's best offering. It was their pick for wireless devices.
Personally I love my setup and It works GREAT in games and everything else. Batteries last approximately 90 days. You can also use rechargables.