How can I use LaunchPad.net packages in 11.04?
You can not add Launchpad itself as a source but you can add specfic PPA's sources for packages.
For instance the Launchpad Project Old and New Python Versions has a PPA settings block where it details the information needed to add that PPA to your software sources
By doing so, the packages listed on this page will be made available to you.
If you can figure out what DEB goes first here is a list of all the DEBs you need for python 2.5: packages Open the 'python2.5 - 2.5.5-8~ppa2~natty3' and you get to see all the DEBs involved!