I found the problem!!! See this: http://otkfounder.blogspot.com/2007/11/solving-reportviewer-rendering-issue-on.html You basiclly have to add the following line to your Web.config under the System.WebServer Handlers.
I just migrated to Windows 2008/ IIS 7. Now ALL of my ReportViewer objects are coming up blank. The only thing that shows up is the tool bar, and the toolbars icon images are all litlle red Xs like the images can not be found. (See attached screenshot) Please HELP!