The ArcGIS Mobile walk through for creating a first project extension returns the error "the calling thread must be STA, because many UI components require this" when creating a new MenuItem object.
Add the following line above the System.Windows.Controls.MenuItem ami = new System.Windows.Controls.MenuItem();if (!MobileApplication.Current.Dispatcher.CheckAccess()) { MobileApplication.Current.Dispatcher.BeginInvoke((System.Threading.ThreadStart)delegate() { Initialize(); }); return; }