I installed python 3.3.1 on ubuntu lucid and successfully created a virtualenv as below
virtualenv envpy331 --python=/usr/local/bin/python3.3
You can add this to your .bash_profile or similar:
alias mkvirtualenv3='mkvirtualenv --python=`which python3`'
Then use mkvirtualenv3 instead of mkvirtualenv when you want to create a python 3 environment.
mkvirtualenv3
mkvirtualenv