Had to bookmark this older post from ScottGu: 

Tip/Trick: Cool UI Templating Technique to use with ASP.NET AJAX for non-UpdatePanel scenarios - ScottGu’s Blog

Scott first runs through letting the AJAX ScriptManager control generate JavaScript proxies for accessing server side code asynchronously. I didn’t realise that it was quite that easy!

The rest of the post covers a way of using this feature to render templates to pages. By using ASCX controls as templates this gives you an interesting way to separate controller and view code.