I installed python 3.3.1 on ubuntu lucid and successfully created a virtualenv as below
virtualenv envpy331 --python=/usr/local/bin/python3.3
I added export VIRTUALENV_PYTHON=/usr/bin/python3 to my ~/.bashrc like this:
export WORKON_HOME=$HOME/.virtualenvs
export VIRTUALENV_PYTHON=/usr/bin/python3
source /usr/local/bin/virtualenvwrapper.sh
then run source .bashrc
and you can specify the python version for each new env mkvirtualenv --python=python2 env_name