Here's a commit message that showed up in the main Rails repository earlier today:

BACKWARDS INCOMPATIBLE: Renamed application.rb to
application_controller.rb and removed all the special casing that was in
place to support the former. You must do this rename in your own
application when you upgrade to this version [DHH]

Well, getting rid of special casing is great, and this commit does indeed simplify some of the Rails internals a bit. But I can hear some of my readers cursing: this seems like an arbitrary change just when Rails 2.2 is about to come out.

Fortunately, if you're moving to Rails 2.2, this won't affect you (yet). We're close enough to 2.2 final that the main repository has been branched: there's now a 2.2 stable branch (which is currently accepting very few bug fixes) and a master branch that's targeted at Rails 2.3, or perhaps Rails 3.0. This change - and some other big changes that have been committed over the past few days - is on the master branch.

So, for Rails 2.2, don't panic. On the other hand, if you're tracking edge Rails closely, the next few weeks are likely to be a time of vast change (and instability) as some pent-up major changes hit the repository. Be sure you know what you're cloning before you set up a new Rails application.