Here's some more interesting reading Glenn... this sucks.

"You can't have your app directly take over file associations anymore in Windows 8. There are guidelines for how to handle this for both Windows Store and desktop applications"
http://stackoverflow.com/questions/14238...ly-in-windows-8

"In Windows 8, apps no longer have the ability to set, change, or query the default handlers for file types and URI scheme names. When developing a Windows 8 desktop application version of your app, we recommend that you remove any user interface elements related to these functions. Instead, we recommend that you link to Set Default Programs in Control Panel."
http://msdn.microsoft.com/en-us/library/windows/apps/hh700321.aspx