In the Stock Trader RI there is an implementation of this (one for Silverligth and other for WPF). The main difference between these two implementations is the control that they are displaying.
The Reference Implementation uses the RegionPopupBehavior for managing this and it is creating the PopupDialogActivationBehavior (Silverlight) and the WindowsDialogActivationBehavior (WPF). Therefore, you could to create your own implementation of DialogActivationBehavior
for achieving your scenario.
For more information on this topic, you could take a look at the following code files:
Hope this helps.