.NET 4 Task/Async Support in Service Clients
Upgrade all projects to .NET 4.0 and change Async APIs on the Service Clients to use .NET 4.0's Task/Future so it can work with C#'s await/async
All Async ServiceClient API’s now return Task with all tests passing.
Dylan v.d Merwe commented
In the mean time, please see my fork that supports this for your use: https://github.com/dylanvdmerwe/ServiceStack