Laggy Magic Mouse with Macbook Pro
I've had the same issue, starting with a late release of Snow Leopard, and now with Lion. The mouse works fine for a while and then it gets really laggy. Turning the mouse off (disconnecting bluetooth with the computer) and turning it back on fixes the problem, but it has happened at least a dozen times in the past week or so.

I've reported it as a bug to Apple, but it is hard to reproduce. I also have a MacBook Pro with an external monitor. I'm not sure if that is just coincidence, though.