There is /usr/bin/python, which is usually earlier in the path (has a higher precedence) than MacPort's binary folder (/opt/local/bin/python). The python_select script changes only /opt/local/bin/python, so you have to make sure there is no other python binary on your path taking precedence over it.
Solution (as root):
mv /usr/bin/python /usr/bin/python.orig
ln -s /opt/local/bin/python /usr/bin/python
port select --set python python26
Make sure you replace python26 in the third line with your intended Python version.
It worked for me a few days ago with MacPort 2.0.0. If you have an older MacPorts library, then you should either upgrade it or use python_select instead of port select --set python.
