If I can cast a vote...
I note that Python 2.6 is the lowest version of Python that's supported, some recent data might suggest that Python 2.6 support isn't so useful in the wider ecosystem and so might be slowing Spark development. A "Python 2 vs 3" survey was conducted before Christmas, the results are recently in:
Of 6,746 respondents less than 10% use Python 2.6 day-to-day. 81% use Python 2.7 (and 43% Python 3.4 - including me) for day-to-day use (presumably for work), there's an approximate 50/50 split between Python 2 & 3 for personal projects. I'd humbly suggest that supporting Python 2.6 will slow development and avoiding Python 3.4 will hinder winder adoption.
The same survey a year back had 4,790 respondents, the second diagram on randalolson's site compares 2013 to 2014 - fewer people now are writing Python 2 day-to-day and more people are writing Python 3 (though Python 2.7 is still significantly dominant). Given that Python 2.7 will be deprecated by 2020 the trend to Python 3.4 is clear. Core scientific libraries (e.g. scipy, numpy, pandas, matplotlib) all work in Python 3.4 and have done for several years.
The survey doesn't ask respondents whether they are web-devs, data scientists, ETL-folk, dev-ops etc so it is hard to extrapolate whether Spark-users are predominantly Python 2.6/2.7/3.4 but I'd suggest that a local survey in this community might provide useful guidance.
Although it is on a longer cycle the major Linux distros like Ubuntu are switching away from Python 2.7 to Python 3+:
https://www.archlinux.org/news/python-is-now-python-3/ # switched 2010
http://www.phoronix.com/scan.php?page=news_item&px=Fedora-22-Python-3-Status # Fedora to Python 3 around May 2015
https://wiki.ubuntu.com/Python/3 # work on-going, maybe the switch occurs in 2015?
What is the use case for Python 2.6 support? Personally I'd vote for supporting 2.7 as a minimum with a strong push for Python 3.4 compatibility to reduce wasted hours supporting older Python versions. Supporting older Pythons will also hinder the creation of a Python 2.7/3.4 compatible code-base due to cross-language complications.
About me - long-time speaker/teacher at Python conferences, O'Reilly author (High Performance Python), co-org of the 1000+ member PyDataLondon meetup and conference series, Python3.4 proponent since April 2014. At my PyData meetup I regularly query my usergroup (approx. 100 attendees each month), 1% use Python 2.6, the majority use Python 2.7, each month more people switch up to Python 3.4 (mainly to get away from unicode errors during text processing).