Just wanted to describe my experience with deploying an app built on 2.4 to jboss 3. After stuffing around with the datasource config for a little bit, everything worked nicely without any redeployment.
I wanted to mention this because I have been a bit scared to look at 3 as an option fearing too many changes. However I didn't even have to resort to the manual which I think is a big well done to all the jboss guys.