I've built a prototype using ExtJS called opsoro. Opsoro is a RIA client for the leading Open Source Enterprise Content Managment System (Alfresco).
You'll find more information at the opsoro wiki site: (http://wiki.alfresco.com/wiki/Opsoro)
There is also a link included to a short screencam.
Alfresco has a very nice RESTful Interface that I'm using for a JSON based client-server communication.
Opsoro is available under GPL and I'm now searching for some additional contributors...There is a lot of stuff to do! :-)
it seems that we'll support AIR or "normal" browser environments in the future. But I can't promise because we haven't the resources to do further development without sponsoring from customers. So we'll move in the direction customers or contributors push us.
I'm not sure what you mean by end-to-end prototype...
Perhaps this http://www.opsoro.org/architecture gives you more information about opsoro and how its cooperates with alfresco.