Initial setup (only do this once)
On the ubuntu box, install esound. This will automatically remove two packages (pulseaudio-esound-compat ubuntu-desktop), but they don't seem important.
$ sudo apt-get install esound
On the Mac, install the SoundFlower package via its GUI. Then install the esound on the Mac from the command line (I use MacPorts for this stuff, and so should you.).
$ sudo port install esound
Streaming audio from Mac to Ubuntu (do this every time you stream audio)
Start the esound process on the Ubuntu box. It will receive audio that has been broadcast over the network from any local computer.
$ esd -tcp -public
Then on the Mac, open System Preferences and set sound input and output to Soundflower (2ch). This will capture the audio from iTunes and make it available to the command line.
Start the esd audio process up (do this before you start CAPTURING sound).
$ sudo esd -tcp -bind ::1 &
Then capture the audio (with esdrec) and send it (that's a pipe character) over the network (esdcat) to the Ubuntu box (mine is called harold.local).
$( esdrec -s ::1 | esdcat -s harold.local ) &