Unity Technologies, maker of the ubiquitous gaming engine of the same name, will be "skipping" Windows Phone 7, according to their CEO, David Helgason. He explained that the decision was made based on the fact that WP7 is closed environment:
"[WP7] is a relatively closed system so you can't run native content, which means we can't really support it. We could, in theory, do what we've done with Flash, which is to rewrite the engine so it works inside a non-native environment. So it could be done, in theory, but it's very much in theory because that would require a huge amount of work and uncertain results."
The good news is that Unity is taking a close look at Windows Phone 8, hoping that it will be more open and better suited for their content. In the meantime, Microsoft will be missing out, while its competitors, iOS and Android benefit from the hundreds of apps created with Unity's engine.
This is not the first time that WP7 has been bypassed by a major gaming engine. Epic Games put forth an effort to port their Unreal Engine over to Windows Phone, but Microsoft refused to give them native access, despite a very amicable relationship between the two companies. While it is understandable that MS wants to avoid opening up their OS completely, it could be a bit misguided to cut off these opportunities, especially considering that they are heavily pushing Windows Phone as a mobile gaming platform.