FYI, I wanted to modify ical4j-connector to have a better model of collection relationships and get away from httpclient 3.x and JackRabbit (the JackRabbit made it clear that they won't move to httpclient 4.x soon). But I came at the conclusion that it would have to be a complete rewrite.
Kerio made available a small CalDAV connector on GitHub, and I'm adding the missing parts on it. See https://github.com/Kerio/cdav-connector
Right now, it doesn't have all the features of ical4j-connector, but it will soon (I'm working on the calendar-query part this week). And no worry, it uses ical4j to parse iCalendar objects too.
If you are going to use it, use the DavStore class! The HTTPSConnection only works for Kerio Connect. I'm testing the connector against iCal Server 10.8, Kerio Connect 8.1, Google Calendar, Fruux and CommuniGate Pro 5.4, so it should work with most CalDAV servers out there.