Avoid Ubuntu 11.10 in production for a couple more months to allow bugs to be found and resolved.
Yesterday we learnt this the hard way when we re-deployed a site that has been running on Ubuntu 11.04 (Natty) for the last 3 months to an Ubuntu 11.10 (Oneiric) instance. By the next day we had already identified 2 major issues:
PHP's session cleanup cron job was using 100% CPU by forking fuser for each session (forum), causing 503s to users. We reverted to the 11.04 version of the cron job.
Apache logs were full of "408" (client request timeout) errors, which we still haven't diagnosed.
There may be others lurking, so we decided to go back to 11.04. Fortunately our deploy scripts made it relatively quick to provision 11.04-based instances to swap into the load balancer.