Matt Quinlan of Appcelerator visited the Denver Open Source Users Group for our October meeting and gave a great presentation on how Appcelerator is an abstraction layer from your choice of backend web service provider (Java, PHP, ruby, .Net) and also provides a tag library that gives you access to the best of Prototype, JQuery (coming soon), YUI, Scriptaculous, and more.
The best part is how easy it is to try out Appcelerator. Just load up this page and start playing. You'll be hooked in no time and ready to download the SDK installer for your platform of choice.