I undestand the original poster's request. It sure would be handy to have a professional and polished "easy to integrate" messagewindow replacement leveraging telerik's controls.
Certainly we can create a user control with the provided controls but an out of the box control would be a great addition!
Something in this spirit:
http://silverlightmsgbox.codeplex.com/ but more polished / finised.
Thanks Telerik!