Good news: async/await is coming to Windows Phone 7.5 development.
Via: http://blogs.msdn.com/b/bclteam/archive/2012/10/19/using-async-await-without-net-framework-4-5.aspx
Do you want to use await but don’t want to wait until you can target .NET Framework 4.5? The waiting is over and awaiting is about to begin.
Today, we are proud to announce an update to the Async Targeting Pack we had previously released. The previous targeting pack allowed you to use await when targeting .NET Framework 4.0 and Silverlight 5. Our updated targeting pack allows you to use await in Visual Studio 2012 when targeting any of the following platforms (or higher versions):
- .NET Framework 4.0 (with KB2468871)
- Silverlight 4
- Windows Phone 7.5
- and portable class libraries targeting those platforms
So effectively we added support for Windows Phone 7.5, Silverlight 4, and portable class libraries.
Sascha Corti aka. TechPreacher 
Wait a second… I thought Windows Phone 7 development was not allowed in VS 2012?
If I try to open my WP7 projects in VS2012 it says “unsupported project type”, so…? What should I do?
This is in the works. Just have a little patience until it gets released.