If you have a localized application and use Gettext, you probably use some method to handle localized Date.
Mine was a widely used approach consisting of redefining various Date.rb constants (Date::ABBR_DAYNAMES, Date::MONTHNAMES, etc.) to use the Gettext Object#_() method.
Recently I was forced to use Windows™® as my primary work OS, so I installed latest Ruby One-Click Installer which comes with Ruby 1.8.6. (I’m running 1.8.5 on all other machines).
This is where I was bitten by the Ruby 1.8.6 Date change, all my lovely constants are now frozen and I can’t localized them anymore (at least so simply).
So stick with 1.8.5 as long as you don’t really need to upgrade, there are also some threads issues with this release… so you also have weird issues if you use backgroundrb.