One of the biggest problems (least in my opinion) with postbacks in the current version of ASP.NET is the user sometimes get a surprise when the page reloads in front of their face when they click on something... sometimes in the middle of a scroll or another data entry. When the page reloads, the user has lost their scrolling position.
Nikhil's way involves using the event that currently requires a postback to create a XMLHTTP request on the client, send to the server for processing (the postback), and then use the client script to update the UI based on the XMLHTTP response. This method is basically a callback. IE and Mozilla already have XMLHTTP in them... too bad Netscape doesn't (is that still around?). Pretty cool implenentation.