On Mac OS X 10.5.8 Leopard, after running the 2.6.x dmg installer from Python.org:
virtualenv --no-site-packages newenv -p /Library/Frameworks/Python.framework/Versions/2.6/Python
Noting of course that 'newenv' is arbitrary. In fact, the syntax in the other link from iElectric is better since it puts the new name at the end:
virtualenv --no-site-packages --python=/Library/Frameworks/Python.framework/Versions/2.6/bin/python newenv
...
An aside: although that seems to work, I get errors due to some mixup with the 2.5 that's installed by default. Not sure if that matters:
/usr/local/bin/virtualenv:5: UserWarning: Module pkg_resources was already imported from /System/Library/Frameworks/Python.framework/Versions/2.5/Extras/lib/python/pkg_resources.pyc, but /Library/Python/2.5/site-packages is being added to sys.path
from pkg_resources import load_entry_point