While participating in the Autodesk discussion groups, Geoff Williams noted that a recent Microsoft patch updated his Internet Explorer such that he has to press the SPACEBAR or ENTER to use the Autodesk DWF Viewer or Autodesk Design Review when accessing a DWF file embedded in an HTML page.
Jimmy Bergmark has a nice recap on his JTB World blog.
A partial workaround has been developed and documented at therippa.blogspot.com.
Autodesk discussion group participant, Steve Segarra, even provided a before and after sample HTML using a similar technique. You can download a zip file with a sample DWF file and sample HTML pages. Download DWFViewerActiveXApril11SecurityUpdateProblemExample.zip (28KB).
Autodesk Software Developer, James Macbeth, also created a solution to demonstrate a workaround. Download TryHTML.zip (6KB).