Windows Phone 7 is currently (v0.9.1) one of our main target platforms we use for developing and testing our games, samples and tests. We have worked on Windows Phone 7 a bit in 2010, see our ZombieParty game released in 2010 in the first weeks after the WP7 came out, but most of our time is spend on the other platforms. While it is very easy for a .NET developer to just develop for the Windows Phone 7 (in XNA or Silverlight), your features and your code itself will get very limited to this particular platform, which is something we don't want for the Delta Engine.
Instead of just developing directly for WP7, we do all development on Windows and then try to bring everything back to WP7, iOS, Android and all the other platforms via the Build System, which usually means we need to go through a lot of extra steps, handling all the content ourselves and providing tons of fallback code for unsupported features. In the end the benefits are pretty cool, applications just work out of the box on all platforms :) Contact us on the Marketplace site if you want to deploy to WP7 (all you need is a WP7 developer account from Microsoft).