I know Silverlight OOB apps support them but what is the case for full trust in browser?
So for example you could click on a stock link in a trading application and we would pop out window that shows you some details about the stock price such as realtime updating charts etc…
We want this as a true window not a Javascript popup or a ChildWindow in SL.
From this blog (linked from silverlight.net): http://10rem.net/blog/2011/04/13/silverlight-5-working-with-operating-system-windows and the official Silverlight blog: http://www.silverlightshow.net/items/10-Laps-around-Silverlight-5-Part-6-of-10.aspx it seems it’s an OOB only feature.