Just wanted to throw in my experience. I had a Mac Mini 1.8 2008 model and a Macbook Pro late 2006 model. I had 1 Microsoft Bluetooth 5000 mouse primarily for my Macbook and I loved it. Worked flawlessly. I started using it on my Mini because I liked it better than my Mighty Mouse, and it also worked flawlessly. So I bought another one just for the Mini.
I upgraded my Mini to the 2009 model and the cursor was chunky and had a stuttered response. I tried both mice on the Mini and the Macbook, both worked perfectly on the Macbook, and terribly on the Mini. Both had the same version OS.
I noticed through troubleshooting that when I restarted the machine, it was smooth, but then bacame stuttered in movement. It seemed when I turned off my wireless keyboard, aluminum version, the smoothness returned, but not every time. So I tried pairing my keyboard and mouse to my macbook and they worked just fine.
All signs point to the hardware/firmware. However, there are no problems with my wireless Mighty Mouse on the Mini.