How do I get System.Windows.ShowDialog() to return ‘true’?

http://msdn.microsoft.com/en-us/library/system.windows.window.showdialog.aspx

ShowDialog returns a Nullable<Boolean> value that specifies
whether the activity was accepted or
canceled. The return value is the
value of the DialogResult property
before a window closes (see
DialogResult).

Basically, you decide by setting the value of the DialogResult, not by hitting a particular button — you decide what the button does.