It's possible to test IE6-8 on Mac OSX via free virtual machines that microsoft offers and then running them in something like VMWare Fusion. http://www.microsoft.com/downloads/en/details.aspx?FamilyID=21eabb90-958f-4b64-b5f1-73d0a413c8ef&displaylang=en
However, the IE9 image does not seem to be out yet. Does anyone know anyways which we can test it on a Mac OSX machine?