In quantal, python3 is supposed to be the default version of python. However, it seems that e.g. python-matplotlib package is only packaged for python 2.7 (and there is no python3-matplotlib). What is the transition plan? When is python3 going to be fully supported, including major packaged 3rd party modules?
While Python 3.x will be included by default in Quantal, it does not mean it will be the default (/usr/bin/python will remain python2, and without python2 installed will not exist). Also, as you have noticed, all upstreams of python packages have not added support for Python 3 yet.
For all upstreams to be able to support Python 3, it is going to be a very long and arduous journey. Even while Ubuntu may have only python3 installed by default, it doesn't mean python2 won't be used by other applications, or that all packages will support it.