Using WPF, Virtual Earth, and WPF/E together...
The next problem you will very quickly discover (or read about in the "Supported Scenarios" link) is that a "hosted Windows Forms control is drawn in a separate HWND, so it is always drawn on top of WPF elements." Oh oh, that means you can't overlay WPF elements over the map.
WPF/E is a perfect workaround to the WPF/Windows Forms interoperability limitation.
As you can see, this approach even works in 3D mode!
You can download the sample code here.