Adding PPA repositories when you're behind a firewall

I cam across this problem when I wanted to upgrade to Firefox 4 on Lucid (10.04). When you add the Firefox repository, it doesn't complete properly because by default it usses port 11371, which is of course blocked on most corporate firewalls! To get around this, you need to edit the following file:

 /usr/share/pyshared/softwareproperties/ppa.py

and change the entry for keyserver.ubuntu.com to hkp://keyserver.ubuntu.com:80 on line 84. Then GPG keys will now be downloaded over port 80. Woohoo!