I got the same error in my virtualenv when I upgraded from 11.10 to 12.04
Turned out to be the python2.7-minimal didn't get properly upgrade, so /usr/bin/python is 2.7.3, but /usr/bin/python2.7 is 2.7.2
`aptitude reinstall python2.7-minimal` then recreate the virtualenv fixed the problem.
I got the same error in my virtualenv when I upgraded from 11.10 to 12.04
Turned out to be the python2.7-minimal didn't get properly upgrade, so /usr/bin/python is 2.7.3, but /usr/bin/python2.7 is 2.7.2
`aptitude reinstall python2.7-minimal` then recreate the virtualenv fixed the problem.