If you have an intel mac, your can create a virtual box using Sun's free soft. This basically let you install a virtual "Windows machine" on your mac. Just get an old windows 95 from a PC friend (you do not need any fancy, memory hungry vista from testing purpose) and install multiple IE (do not install IE7 first, you won't be able to install 6... install 6 and update to 7 if needed... you can always revert the upgrade to test IE6), FF, Opera and all to run tests on all those if you are serious about cross-platform compatibility. You can also use bootcamp, but as far as I'm concerned, I just want to be able to quick test my CSS, so a virtual machine is way faster than having to reboot.
Browsershots can be usefull, but it is slow, so I use it to test only after I've done a local battery of tests and then check the odd browsers...
Like Iwakami mentions, a test on a real machine is the best thing. If you want to get serious about your thing, buying a cheap PC/a mac mini for PC users... could also be worth it. (I'd like to have a try on those EEE PC by Asus to see if it can be a serious test platform)
Like Vaska said, IE is far from being standard compliant... But I would say that is possible to do great web design that will look good in IE and all the other browsers. And use the extra things you can get from standard compliant browsers to push things up a notch. This will hopefully make hte IE users change to other browsers to get that extra thing you've done, while still offering a decent experience to IE users (which often have no idea of what browser they are using, for good or bad).
Personally, I think that, from my experience, Indexhibit does gracefully degrade with IE. So the the issues with sites looking different on Mac vs PC are the sole responsibility of the designer.
Sorry for the long post ;)