WordPress 3.4 will have a brand new theme in it – Twenty Twelve.
The initial version of which, has just been added to the development version of WordPress. The commit message explains the philosophy of Twenty Twelve:
Initial import of the Twenty Twelve theme, by Drew Strojny. This time around we’re trying something different than the previous twenty-something themes, a much more minimalist approach that affords easy use as a CMS in addition to being blog-forward. More information will be on wpdevel soon. Props drewstrojny and lancewillett.
Source: Trac Ticket #19978
I’m very interested to see how it pans out!
Update 7/2/2012: The Twenty Twelve Development Team has posted an update:
This is not yet a fully working theme — we’ll be adding in more features and lots of missing styles over the next 3–4 weeks. Including post formats, comments, archives and page templates, more in-post styling, and a nav menu rework so the main content comes ahead of the navigation.
Update 2/3/2012: The Twenty Twelve theme has been delayed, and will no longer be part of WordPress 3.4. It will be in WordPress 3.5 (scheduled for later in 2012).