I'm using Arch and the transition was smooth for all the packaged libraries including Numpy, Scipy, matplotlib, and friends, as well as pure Python packages like mpi4py and petsc4py which I install from upstream since I track development versions. The real pain is for projects that are not pure python libraries, but want to ship portable scripts. Since the "python2" link is not ubiquitous, they cannot put "/usr/bin/env python2" in the scripts (like Arch packages do, perhaps just using sed). Unfortunately there are still modern installs of Python that do not have "python2" links, so this is going to be a long process when it's not feasible to support both python-2.x and python-3 within an identical codebase. That is challenging if you have to support e.g. python2.3 which will be around for a while longer (RHEL4). Also, RHEL5 (rather, the CentOS system I have a login on) does not have a python2 link (it has python2.4) and that will be around for another four years.

I think Arch's decision was a bit premature, but waiting isn't going to make the issues much less painful if your project will always have to support an 8-year age span of distributions.