Scott Guthrie announced the Atlas project in a recent blog post. The Atlas project aims at considerably simplifying and enriching the client script development experience. It includes but is not limited to AJAX/callbacks and cross-browser client-side declarative binding of elements and behaviors. One of our goals is to leverage the experience of client-side programming that exists inside Microsoft (see Outlook Web Access, MSN Spaces, MSN Virtual Earth, etc.) and give easy access to that technology to developers.
One thing that's certainly not that clear enough about callbacks is what happens with regular postback events and control state. During a callback, we reconstruct the state of the page as it was during the last postback. The reason we are doing this is that we want the logic in the callback method to be able to access the rest of the page with controls in the right state.
Did you ever notice how people can't ever throw a marker away? How many times have you seen this scene?