When running bin/tox with pip 1.4.1 (and pyrun) you get:
Could not run 'bin/tox': No module named req_file
Looking into the code, this line is causing it:
lib/python2.7/site-packages/tox/_config.py: -- from pip.req.req_file import parse_requirements
The current pip does come with a req_file module. It was added in Jan 2014 and released in version 6.0 of pip.
The setup.py of tox does not define a dependency on pip:
I guess it would be good to define the minimum version with which tox works.