<p>I'd like to create a popup dialog box in silverlight  in which i can manipulate controls, enter data, and return a value. I want it to be modal, so that when it is open, the page "Below" is inaccessible. I havent found an easy way to do this yet. Any suggestions?</p>

I haven't found a perfect solution either. The closest I have seen is this:
Using Popup to create a Dialog class
If it is ok to be non-modal, you can try this tip using HtmlPage.PopupWindow().
How to Popup a Browser Window