OK!
I figured out how to install Pd-extended on Ubuntu Intrepid:
*THIS IS A DREADFUL, UGLY HACK. EXECUTE AT YOUR OWN RISK*
via the command line, do:
you@machine:~$ sudo dpkg -i --ignore-depends=liblame0 Pd-0.42.0-extended-<date>-ubuntu-hardy-i386.deb
then in your favourite text editor and using sudo, edit the /var/lib/dpkg/status entry for pd-extended. Where the dependencies are listed (Depends: ) change any reference to liblame0 to libmp3lame0. This will stop apt from seeing pd-extended as broken.
Make sure the 'Depends:' line remains on a single line in your editor - I found this out the hard way!
You will have to redo these steps every update.
Chris