MSDN has a new article up today: An Overview of the New Services, Controls, and Features in ASP.NET 2.0. I'm jazzed up about all of the many new features to come, and this one is going to be very, very nice for a lot of intranet apps (most of the companies I contract for standardize on IE)...
ASP.NET 2.0 introduces a client callback manager that permits pages to call back to the server without fully posting back. Callbacks are asynchronous and are accomplished with XML-HTTP. They don't include postback data, and they don't force the page to refresh. (On the server side, the page executes as normal up to the PreRender event, but stops short of rendering any HTML.) They do require a browser that supports the XML-HTTP protocol, which generally means Microsoft Internet Explorer 5.0 or higher.
