Super Preview which will allow you to preview you pages in Internet Explorer 6, 7, and/or 8 was announced today at MIX 2009 and on the Expression Web Team Blog.
How many times have you had to debug your web pages on virtual or multiple machines running different versions of Internet Explorer? Or had to wait for a slow web service to return renderings of your pages?
You don’t have to do that anymore. Now you can debug your pages on multiple versions of IE on the same machine that you use for Web development.
Expression Web SuperPreview for Windows Internet Explorer is a stand-alone visual debugging tool that makes it faster and easier to migrate your sites from IE6 to IE7 or IE8. With SuperPreview, you can ensure that your Web sites work correctly in IE8 while also maintaining compatibility with earlier versions of IE.
SuperPreview shows your web pages rendered in IE6 and either IE7 or IE8, depending on which version you have installed on your machine. You can view the pages side by side or as an onion-skin overlay and use rulers, guides and zoom/pan tools to precisely identify differences in layout. You can even compare your page comp to how the targeted browsers render the page.
Additional Information on SuperPreview from the Microsoft Community Site.