I do hope they improve their policy on regression-tests and upgrade instructions. 3.0 -> 3.1 wasn't very smooth for me, the bugs were just more subtle.

My favorite bug during the upgrade: Someone decided (long ago) it'd be a good idea to store a Marshalled object in the session-cookie. One class serialized therein disappeared in Rails 3.1. Result? Any and all requests with a session-cookie bomb out with an unmarshall error.

tldr: Make sure to rename your session cookie before rolling out the upgrade. This logs out all users but is still more desirable than the alternative...