Atlas is a framework for building AJAX style web applications. One of the big announcements at the conference was a new CTP with a go live license. Due to popular demand, this license will allow developers to put Atlas applications into production.
You can download the March CTP from here. Some of the highlights in the new build include enhancements around web parts, enhanced support for web services, and the ability to build gadgets for Live.com. You can find a list of all of the changes here.
-Marc and David