Out of the box, the ASP.NET Ajax UpdatePanel and UpdateProgress controls do a pretty good job of removing postbacks from your applications, but they need a little bit of styling to get them looking their best. Here's a couple of links to get you started on the right track:
Link: Using the UpdateProgress as a modal overlay
Link: Gmail style 'loading' indicator using UpdateProgress
Link: UpdateControls: UpdateHistory and AnimatedUpdatePanel
Raj's modal overlay (the first link above) allows you to target either an individual control (such as a panel or a GridView), or the entire page. It's very slick looking, but it has issues in IE7. The issues have been pointed out the comments, so I'm hoping a fix isn't far off.
Also while reading I noticed this post which mentions that the 18.104.22.168 release of Firefox broke many Ajax Frameworks. The issues are fixed from 22.214.171.124 onwards, but it might be worth bearing in mind if you or your users or testers are having Ajax related issues in Firefox.