See: 8322f5d#commitcomment-1629476
Building on Lion fails, see:
http://pastie.org/4331159
This appears to be because ${buildout:location}/parts/buildout is injected into PYTHONPATH, which seems incorrect for purposes of what virtualenv needs to do -- site.py is a monkey wrench here.
At the moment, I am not sure what to do but use an older bootstrap.py -- which works.
Sean