See YoLinux: Linux Audio Streaming as one option.
This also might be useful: Streaming audio on GNU/Linux - the miniest HOWTO.
Airfoil Speakers for Linux is here, but this is only a small part of an answer.
From Anything like Airfoil (audio stream util) that can run on Ubuntu:
I use raop-play.
I mainly run debian unstable and got
it work on my laptop with ubuntu junky
;-) as well ... I run the stand-alone
client raop_play, not the alsa device.
I got the sound device to compile with
my kernel, however, I never got the
alsa sound device to work ... programs
complain it is no alsa sound device
So what I do is the following:
raop_play airport_express_ip ~/sound.wav &
in vlc I set output to the file: ~/sound.wav
Everything I play through vlc now
plays on the speakers ... I have to
restart both vlc or raop_play about
every 5 hours or so, because one of
them seems to have a problem after
some time (raop disconnects or vlc
hangs) ... not really a big issue,
though ... I use it all day long every
workday ... ;-)
It is more reliable than iTunes, which
just stops sending the audio to the
airtunes every 5 to 10 songs ... I
think my airtunes maybe to blame, but
raop manages much longer connections