I usually don't get excited about such things as a mouse, but I picked up one of these Kensington SlimBlade Bluetooth mice on a whim, and it's awesome. First of all it's wireless, and automatically connects and disconnects from my mac when i go through various states of suspension and re-animation. Second, it switches between a two button mouse with a scroll wheel, and a trackball with one double-click of a special button. Third, it just feels great.
Kensington SlimBlade Trackball Bluetooth mouse
I could see me getting 2 or 3 of these for various computers I use.