Just in case you missed it: 
http://www.theserverside.com/news/thread.tss?thread_id=33803

It may be interesting to post some informed answer about Jackrabbit, 
without of course going into a flame with the eXo people.

<rant>
BTW, these dual GPL/commercial licences like eXo make me puke. These are 
plain commercial entities surfing the marketing wave of free software to 
sell their stuff in a traditional way. I have nothing about classical 
runtime licences, except when they're hidden behind GPL smoke.
</rant>

Sylvain

-- 
Sylvain Wallez                        Anyware Technologies
http://apache.org/~sylvain            http://anyware-tech.com
Apache Software Foundation Member     Research & Technology Director