Re:lesliet
Some KVMs have PS/2 mouse/kbd emulation for the computer that's not directly connected to mouse/kbd (prevents some computers from hanging when the devices are switched around). There is some messages being passed around. I'm not sure if USB will function in a similar manner - USB won't have the computer hang problem, but there may be other traffic on the ports.
If you don't mind going headless on the PC, you can install VNC on the PC, and use a viewer on the Mac. I use UltraVNC ( ultravnc.sf.net/ ) which has some nice features. It's not the most friendly to set up (with the mirror display driver) but once it's done, you never have to fiddle with it.
The Fairly Complete Mac Mini Switcher's Hardware Guide