I'm always getting grief from my managers about not testing in IE6, which is hard when IE7 is installed on all machines these days. I recently found a little install tool which allows you to run multiple versions of IE on the same machine. It doesn't fully install IE, just the dll's and uses dll redirection to trick windows into running an older version of IE.
Although it's not perfect, it's a lot easier than running a virtual PC for each version.
The install can be found here: http://tredosoft.com/Multiple_IE