Checking the CSS compatibility between browsers is a hassle. Especially when you have to check for different flavors of the same browser. I was using VPC images to test various IE versions which is a real pain. I ran across this while trying to find a website that did compatibility checking.
IETester. This is an alpha release of a browser that can test your CSS in IE5.5 up to IE8. It was pretty accurate too. I did have a couple times that the browser tabs crashed. I am not sure what caused it but that was a minimal issue compared to trying to test with VPC images.
Check it out here: http://www.my-debugbar.com/wiki/IETester/HomePage