Although there is no info about this in the rails blog, I did a gem update this morning, anv voilà:
papipo$ sudo gem update
Updating installed gems...
Bulk updating Gem source index for: http://gems.rubyforge.org
Attempting remote update of actionmailer
Install required dependency actionpack? [Yn]
Install required dependency activesupport? [Yn]
Successfully installed actionmailer-2.0.0
Successfully installed actionpack-2.0.0
Successfully installed activesupport-2.0.0
And it keeps going with all 2.0 libs.
Greetings for the rails team.