I've tried to upgrade our running confluence instance 4.0.3 to 4.3.2 two times. Both failed in the manner that confluence does start, then freezes during first startup. On next startup there is no content rendered - no CSS, no nothing.
Fortunately, the platform is virtualised so rollback saved the day.
Next upgrade window is set for tomorrow, 15.11.2012 at 15:00 GMT+2. If nothing special is done, I suspect the upgrade will fail as well. We use a mysql database with the confluence instance, running on RHEL6 (both the DB and confluence are installed on the same machine). The machine also has ipv4 and ipv6 addressess.