It's a pain - you have one machine and you need to test your site on Mac, Windows and Linux browsers.
Up until now, you could either dual boot your PC to Windows and Linux, or emulate Windows on a Mac. Doing all three from one box, though? Can't be done, right?
Until now. Connectix have launched Linux VirtualPC for Mac which allows you to software emulate RedHat on a running Mac - no dual-booting required.
The cynical would suggest this is the best way to get a stable webserver running on a Mac, but that's not me. Honest.
More emulation info