It's not officially supported by Microsoft. IE8 will have the IE7 compatibility mode, so you'll be able to test IE7 and IE8 compatibility in that one browser.
Personally, I use virtualization to test IE compatibility on my Macs.
That said, there is a hack out there that works well enough... http://tredosoft.com/Multiple_IE
There are different variations of this that can be found through a Google search.