Twenty Twelve, the next great WordPress default theme, has unfortunately been removed from the 3.4 development cycle and pushed back to 3.5. There is no specific reason given for the change, but it seems likely that the team would rather focus on the core functions as WordPress 3.4 draws closer to its inevitable April release.
If you already have Twenty Twelve activated and are using trunk, make sure that you activate another theme before upgrading to the latest, as Twenty Twelve will be removed. If you want to continue to use Twenty Twelve, you can grab it from GitHub for now.
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