Just came across a site that outlines how to get IE 5.0, IE 5.5, IE 6.0, and IE 7.0 all on OSX and run natively using X11.
You'll need to install X11 from the Tiger DVD's you got when you bought it, or head on over to this guy's site to download it. For those on Leopard, you're in luck by default.
So, with IE 5.0 all the way to IE 7.0 now being able to be run on Mac natively using Darwine and X11, there doesn't seem to be much need to get back on Paralells for debugging my CSS and AJAX interfaces anymore.