Darran, question is what major changes happened in Neg to warrant a v3. You can do 2.1 if you wish....
The bigger feature is fallback to FORM authentication is in and the dropping of support for JBoss AS 4 and 5 but calling it a 2.1 could also be sufficient.
True delegation is something coming shortly so maybe that would justify a 3 release.
IMO as and when you have a feature implemented, makes sense to put out a point release. 2.0.4, 2.0.5
It is better to put the feature out into the community asap for feedback.
The versions we have (2.0.x, 2.1.x, 3.x) means more branch headache. So think about v3 only if you really want to.
If it goes into 2.0.4 I don't really think it is good to drop support for the previous AS versions, this is why I think some change in version is needed.
Or just modify the authenticator to retain support for both versions is also possible.
Darran, now that AS6 is out. People can upgrade 5.1 to 6 if needed. So base your code now on AS6 if possible. I think makes sense to do 2.1 with all the changes you are planning.