MessageBox.Show for Windows Phone 7

I need a MessageBox.Show(), but with an extended functionality: the built-in messagebox for Windows Phone 7 won't show arbitrary buttons, just OK or cancel, I need for example YesNo as well. I need a Show method that will return only when the user clicks on a button. I tried to make my own messagebox as popup. The problem is that it is asynchronous as everything in the UI-thread in WP7, and I cannot block the UI-thread because the UI-thread is responsible for handling the button click events as well. I saw a ChildWindow example for WP7, but that was also asynchronous, I must have a Show() method that returns only when the user clicks. Any suggestions? Some kind of DoEvents() could help me a lot, but there is no such method in WP7 :(