I downloaded the platform preview version of IE9 today to test out some CSS support. I ran a few tests through it, with the following results. The picture on the left was generated using Firefox 3.6 (keep in mind that I did have to use the “-moz” prefix in front of all but one style to make it work in FF). The IE content is on the right.
It certainly is possible that IE requires some vendor-specific suffix like Firefox did. However, I did not use any prefix, so the style may actually be supported in some form.
In any case, the only supported feature out of the four I tested was border-radius (and that is actually the one I’m most excited about). The box-shadow would be next on my list. Hopefully that gets put in the next preview.