Office 2003 XML Shell Extension.
I also noticed that Office 2003 must've installed some shell helper for XML files, because when I saved the file to my desktop, the shell peeked inside the XML file and gave me an XML document icon with the Excel icon overlaid across the upper left corner. I wonder why the file opened in my browser on double-click instead of into Excel...[The .NET Guy]

I noticed this as well. Office seems to achieve this cute bit of functionality by inserting a PI into the XML instance document which the shell extension just scans all .XML files for to determine if the file should be opened with a specific Office application. The PI looks like this:
     <?mso-application progid="InfoPath.Document"?>
[Drew's Blog]

Isn't it interesting that ProgIds continue to be used like this, as a component identifier?  Is the use of a ProgId kosher in a .NET world?