This is one of the items that is tested in your Windows 8 App when you use the WACK.
If you are in Southern California, please email this post for more information on getting a token for your app by June 30, 2012!
Leverages file pickers
A Metro style app can call a file picker window to let the user browse their system and pick files or folders for the app to operate on or to let the user save a file using a new name, file type, or location ("save as"). Apps can also use file pickers as an interface that enables them to provide other apps with files, a save location, or even file updates.
Leverage file pickers within your app if you want to interact with user data that is stored in files.
To learn more about file pickers, check the accessing files with file pickers reference and follow the guidelines for file pickers.