With Python 3 being a huge release and backwards incompatible with Python 2, five years after Python 3.0 was unveiled Ubuntu (and many other Linux distributions) are carrying Python 2 as well on their install media. There's still large amounts of open-source software to be ported from Python 2 to Python 3.
Ubuntu is still trying to do away with Python 2 being installed by default on desktop and server (and Ubuntu Touch) images. The Python 3 transition was discussed this past week at the virtual Ubuntu Developer Summit.
For dropping Python 2 from Ubuntu Server, vim, byobu, landscape-client, and OpenStack clients still need to be ported to Python 3. Ubuntu Touch still depends upon the Python 2 Autopilot. For Python 2 on the Ubuntu desktop, there's still many packages to be ported to Python 3 like Hplip, Totem, system-config-printer, Gconf2, etc.
The main packages to be ported to Python 3 for the Ubuntu desktop are found via this Google spreadsheet. For some of the to-be-ported packages there's states along the lines of "probably hopeless" and there's also Canonical-developed software still bound to Python 2 but Ubuntu developers don't feel motivated to port it to Python 3 as items like the Software Center and the Ubuntu Single Sign-On client are expected to be replaced in their entirety soon.
With Canonical likely not being interested in porting many of the upstream software packages to Python 3 and there being a lot of work involved for some of the packages, it's probably unlikely that the Ubuntu desktop in 14.04 LTS will do away with Python 2 but we'll see how it goes for Ubuntu Server or Ubuntu Touch. This initiative is just about removing Python 2 from the install media but would continue to be maintained within the Ubuntu package archive.
More details on the Python 3 transition for Ubuntu Linux can be found via the vUDS session notes and the session video that's embedded below.