I agree Jim. But I do not see MSFT coming forward and specifying which contracts they support. And the version of the apps currently available are pretty much what the users will get when Windows 8 launches in Sep/Oct later this year.
I will write a little developer script that at least scans the installed apps manifests for share target definitions.
edit: created a quick script that lists the apps that support the share contract. https://github.com/pke/metro-targetbrowser
Maybe MSFT could think about a developer oriented part of the app store, where the apps publisher specifies URLs to API related informations of his product. And the store could display the supported share target types right in the store.
You would only see such information when you switch the settings of the store on your machine to "Show development related information" much like the start menus "show admin options".