I too have disconnection problems with Magic Mouse after updating to OS 10.6.3. After doing so I recall seeing "Low Battery" pop up signals on my iMac's screen. After I replaced the batteries my mouse would disconnect sporadically. I have to keep my "wired" mouse connected to go to the Blue Tooth icon to connect the Magic Mouse. Might it be the batteries?
I updated my OS X to v10.6.3 recently. After rebooting, my Magic Mouse seems to be having problems connecting to my Macbook Pro. The System finds the mouse, but when I click on Continue to connect, it fails. Anybody experiencing the same problem?