Mantis is a WinFX based multiplayer game and engine. All of the GUI and rendering is done using Windows Presentation Foundation and the networking is done using Window Communication Foundation. If you are attending the PDC, be sure to check it out at the Windows SDK booth and the Fundamentals Lounge. A technology preview will be available for download on CommNet soon. A public version will be available about a month after the PDC. The goal is to continue development of Mantis and have it ready to ship when WinFX ships. This will be the "replacement" for Terrarium and will allow you to create your own games very easily. I'll blog more about it in the coming weeks including the architecture, how it uses WPF and WCF as well as some of the tips and tricks I've learned so far.
Here are a couple of screenshots from the build that is running at the PDC.
And here is a short video that demonstrates some of the game play.
The video was capturing at ~15FPS, which is why it looks choppy.