This page describes the effort toward a JDOM 2.0. The purpose of JDOM 2.0 is to make use of features introduced in Java 5, such as generics. Unfortunately the generics changes will break binary backward compatibility so we're moving to a new version as well as a new package name: org.jdom2. People who want to use the JDOM 1.1.x branch can continue to do so, but people who want the modern features can use JDOM 2.0. The same program can even mix and match. Yes there's downsides ((http://markmail.org/message/avsyrgfii7z3u6m5) but it's probably worth it.
How much backward compatibility will we break? Only what's necessary. We may maintain source code compatibility so all a programmer will need to do is update their package imports and recompile their project. But if we decide as a group that some changes are so advantageous they're worth requiring code edits, we'll do it.
We'll do the work on GitHub because it simplifies the task of doing experimental branches. I expect a lot of discussion about whether a certain change is worthwhile or not, and GitHub makes it easy to discuss code not just ideas. The JDOM mailing lists will host the discussion. You can also watch for updates on Twitter from @jdomproject and @jdomcommits.
Possible JDOM 2.0 improvements (each should probably get a wiki page):
People who have volunteered to be heavily involved in the project: