The Gaia panel was completely rewritten in Flex and includes optimization features, easier Gaia project management, and a new publish panel which makes developing with Gaia even easier. Gaia 3 also features a brand new version of SWFAddress (version 2.3) that fixes the Firefox 3 OSX redraw bug.
There is also a completely new addition to the Gaia project: the Gaia Framework Site Creator AIR application. This small AIR application allows you to quickly update your project's site.xml and visualize it as a wireframe sitemap.
Some of the features include XML validation, drag and drop re-ordering, reordering of asset load order, editing project properties, and printing a site map. You can download the first version from Wade Arnold's blog posting about the app.