Wow. Karmic actually did something that really impressed me, there is support for Airport express streaming in the standard repos.
To enable it:
$ sudo apt-get install pulseaudio padevchooser pulseaudio-module-raop pulseaudio-module-zeroconf
Then run System -> Preferences -> Pulse Audio Preference. Tick “Make discovereable Apple AirTunes devices available locally”.
Run RhythmBox, or any app which uses pulse-audio as its backend, and play some audio.
Run Applications -> Sound & Video -> PulseAudio Volume control.
Select Airport Express as the output for this app.

