There is a new kid on the block- XNA Magic. It looks like it is the first game engine built from the ground up for Microsoft's XNA technology. I have requested to be part of the beta. I'll report in if I get in.
From their website:
From their website:
Quote
XNA Magic is the first all-in-one game development system designed from the ground up to take full advantage of the brand new Microsoft XNA platform.
Providing an extensible game engine, level editor, particle system design surface, logic editor, script editor and a great deal more, XNA Magic is designed to be both complete and easy to use.
As well as the core engine that is designed to work on both Windows® XP and the XBox 360, XNA Magic features a fully dynamic design time environment that allows you to see changes to all your game components, shaders and assets in real time.
XNA Magic is not a port of an existing engine, it is a complete solution designed around the brand new XNA Framework. It is therefore completely shader based throughout and has no ties to a fixed function past.
What’s more, unlike many engines that require you to modify the engine source, XNA Magic takes a highly granular approach. You can choose to dip in and use whatever aspects you need and no more. Scenes you create in the development environment can be run standalone or exported as a components that you can call from your own code.
Hope I get through!
My concern though is that the games won't be able to run on a Mac. Most probably, they'll work on XBox 360 though, and this is another tool that smaller game development houses (or individuals! 
). Despite being a Mac-only development tool, Unity is able to produce games for both Windows and Mac.