As Blaine and Glenn mentioned, this week the great folks at Patterns and Practices where able to ship a Reference Implementation that for the web client bundle. I love these releases because the P&P folks think about the problems directly from building out real world applications. They take the products we have such as ASP.NET 2.0, ASP.NET AJAX and the AJAX Control Toolkit and fill in any missing gaps to build out real world applications... they include all the source code for what they build and have active support channels for you to get your questions answered.
Here is a brief summery of what the reference implementation shows (seamlessly stolen from Blaine)
I encourage you to take a look and see if this work can help you in your next product.