While I was away on a two week holiday in Morocco I notice that the Visual Studio team have dropped a very welcome SP1 refresh that means you can now play with the Async CTP with Visual Studio 2010 SP1. It also means you can program against ASP.NET MVC 3.0 and use the Aysnc CTP at the same time.
Visual Studio Blog - SP1 refresh announcement - http://msdn.microsoft.com/en-US/vstudio/async
Lucian Wischik's Blog - design changes - http://blogs.msdn.com/b/lucian/archive/2011/04/15/async-ctp-refresh-design-changes.aspx
Official download - http://www.microsoft.com/downloads/en/details.aspx?FamilyID=4738205d-5682-47bf-b62e-641f6441735b
Have fun and remember to provide feedback to the Visual Studio team to make sure you get the Async features you want in C# 5.0.