As you should know, the new Stage3D feature of Flash Player 11 and AIR 3 provides powerful low level APIs to create high performance 3D graphics in Flash, the issue with this API is that it is of an extremely low level nature that makes it extremely difficult for average developers to pick it up and use it in their projects, and the expectation is that frameworks will be built on top of the Stage3D APIs that will be easy to use and are tailored for specific project needs.
Starling is the first framework built on top of Stage3D and is ready for use right now, this framework aims to help Flash developers create GPU powered 2D graphics that run at the high performance provided by Stage3D. The main highlight of this framework is that is uses convention display list architecture which means that you can pick it up and use it without having to spend a lot of time learning how things work.
Starling Framework projects will run on AIR 3 and Flash Player 11, unfortunately you will not be able to create high performance 2D games for mobile using Starling when AIR 3 comes out because the mobile version of AIR 3 does not yet support Stage3D, but Adobe announced that Stage3D will be introduced to mobile AIR in an upcoming update.
You can watch a video tutorial by Lee Brimelaw that showcases how easy it is to use this frame here, and you can also download a PDF handbook on the basics of Starling written by Thibault Imbert at this link. You can learn more about Starling Framework, look at the samples, and download the framework for your own use by visiting its official website.