Just spotted a new Visio addin over at the Microsoft Visio ToolBox that converts flowcharts created in Excel, PowerPoint or Word to Visio based flowcharts...
This free download, which requires Visio 2007 SP1, is available here:
http://visiotoolbox.com/bpm/Default.aspx (see below)
I've just downloaded the addin and it does an excellent job of converting Office shapes in 2003 and 2007 file formats to proper Visio shapes. You can then, of course, add Shape Data or any of other Visio functionality which makes it such a great diagramming tool.
[Update 18.04.12 - This tool, from Visio MVP Uichiro Nozawa, has been updated for Office 2010 (see system requirements at the bottom of the following page). There is also a new location and it can be found at: