I am curious to why you would not want to use a child window, this is an out-of-the-box control that acts like a floating panel. If you do not want to use the child window option you will need to create your own control or use a third party solution.
Thanks,
Paul Leedham
City of Hudson
http://gis.hudson.oh.us/
Looks like Tim Heuer's FloatableWindow (refactored ChildWindow) might work for this...
http://timheuer.com/blog/archive/2009/05/10/silverlight-childwindow-non-modal-refactor.aspx