If I call a WCF service method I would do something like this:
proxy.DoSomethingAsync();
proxy.DoSomethingAsyncCompleted += OnDoSomethingAsyncCompleted;
In the CTP there are factory methods that do the work of turning regular APM functions (Begin/End) into ones that are compatible with the new async keyword, for instance:
Stream s = new FileStream("C:\test.txt", FileMode.CreateNew);
byte []buffer = new byte[100];
int numBytesRead = await Task.Factory.FromAsync(s.BeginRead, s.EndRead, buffer, 0, buffer.Length, null);
So in your case you can do the equivalent and then you'd then call it like so:
async proxy.DoSomethingTaskAsync()
See this thread on the CTP discussion group for more info