Okay, so a few months back I discovered Synergy, and filed it under "neat stuff worth a quick look." Now I've cast off the shackles of my KVM, cleared off my desk, and set up a second monitor. I can now seemlessly switch back and forth between Mac and PC as easily as you can switch between two screens connected to a single computer.
Here's how it works: you hook up computer A (the server) to keyboard, mouse, monitor, and network. You install Synergy, and start the server. You hook up computer B (a client) to monitor and network, install synergy, and start the client. Now, move the mouse connected to computer A past the edge of the screen, and it appears on computer B's screen - and now that the mouse is there, the keyboard is directed to computer B as well. And because it's just a KM (keyboard and mouse) solution, unlike VNC which has video, the connection is extremely snappy - so much so that it's unnoticeable to me.
Synergy is free, open-source, and works on Windows, MacOS X, and Unix/Linux. If you frequently use two (or more) computers, have the desk space for multiple monitors, and would like so save yourself some serious trouble, you should give Synergy a try.