I believe you meant to post this in the Docking & MDI for WPF forum. This is the PropertyGrid for WPF forum.
To answer your question though, there can be any number of "selected" documents when using a tabbed MDI. So there is no easy way to track the "selected" item.
Each group of documents is contained in a TabbedMdiContainer, which has a SelectedWindow/SelectedItem property. But again there can be any number of these TabbedMdiContainer's based on the current layout. Typically, the TabbedMdiHost.PrimaryWindow is used to track the last document activated by the user.