Someone has to take the first step and break the cycle to get the chicken-egg problem undone. Arch has had Python 3 as the default Python interpreter for a couple of years now and it's been working pretty much fine. Many libraries now support Python 3, and I've done full sites in Py 3. Almost all Python scripts I write these days are Python 3. I don't think I've had to downgrade a script that started in 3 down to 2 for a couple of years now. It's as ready as it's going to get.
The groundwork is done, and I think everyone who is going to support Py 3 without any extra prodding has already done so. Now we need the distros to come through and give that extra nudge to the maintainers that are still slacking, or encourage people to replace those libraries that refuse to update.
For my PyCon Russia talk, I pulled down the data for all 44,402 packages (as of May 31). 13.5% of all packages on PyPI support some version of Python 3. 75.5% of the top 200 packages by download count claim to support some Python 3 version (according to their setup.py classifiers). Additionally, 64% of the top 500 support some Python 3 version.
Another interesting thing I saw was that of those 44K packages, 44% of them have seen a release within the last 12 months (representing 82% of the last month's download share), and 22% of those packages released in the last year support some version of Python 3.
The groundwork is done, and I think everyone who is going to support Py 3 without any extra prodding has already done so. Now we need the distros to come through and give that extra nudge to the maintainers that are still slacking, or encourage people to replace those libraries that refuse to update.