In addition to upgrading a few Rails apps to the latest 3.1 release candidate, we also submitted a few RailsGuides patches, found a bug and submitted a failing test. We didn't even have a chance to fix it before grzuy picked it up and submitted the patch.
We ran into a few bumps moving to the 3.1 asset pipeline, but overall the upgrade from Rails 3.0 to 3.1 has been smooth. As for the 2.3 to 3.0 upgrade, Rails provides deprecation warnings in the previous point release, 3.0.9 as I write this, so the upgrade goes a little something like this:
And now…your moment of zen.