Cadalyst has discovered that Microsoft’s just-released Internet Explorer 7 is causing problems with Autodesk’s DWF format. Autodesk blogger Steve Shepard reports that DWF Viewer, Design Review and TrueView are not compatible with Microsoft Internet Explorer 7. Uninstalling Internet Explorer 7 and reverting to Internet Explorer 6 does not restore Autodesk functionality, because the supporting libraries from Internet Explorer 7 are left in place. Microsoft does offer a Blocker Toolkit so system administrators can disable automatic delivery of IE 7.
DWF is a full 3D and 2D exhange format that is built into Revit. For more information on DWF, see the excellent Autodesk site for Desgin Review (formally DWF Composer).
Update: Autodesk have released a HOTFIX for this issue which can be found on the "Beyond Paper" blog. Very interesting.