Dear lazyweb, sometimes things break and you have no clue why things that used to work without paying any attention to them, no longer work. As an occasional user of virtualenv I was used that I could use my system packages in a virtualenv environment. That meant that packages that are hard to install, like MySQLdb, only needed to be in the system install (and they're system specific anyway, usually) and you didn't have to play around with compilers (use the distro package!) and whatnot to get a module in your virtualenv.
Virtualenv 1.7 changes all that. So after a distro upgrade of virtualenv
from 1.6 to 1.7, all of a sudden, your scripts can no longer find system
packages (a.k.a. modules). Because
Made --no-site-packages behavior the default behavior. The
--no-site-packages flag is still permitted, but displays a warning when used.
Thanks Chris McDonough.
See http://www.virtualenv.org/en/latest/news.html
For the behavior I'm used to and usually want, I now need to include
--system-site-packages when building a new virtualenv.