I just installed Ubuntu Studio 9.04 as dual-boot on my Mac and it's fantastic. Pretty much everything works out of the box, soundcard support, low-latency realtime audio, etc. (The only problem I had was a mild annoyance with my wacom tablet, bug 375329.)
Since I want to use jack as my audio subsystem, I launched "JACK Control" (aka "qjackctrl") from the applications menu and pressed its "Start" button. Then I was ready to make sound.
Installing SuperCollider was super-easy too: I installed SuperCollider from the packages at http://launchpad.net/~supercollider/+archive/ppa and scvim worked straight from its little menu icon.
Darn it, this is easy
(A rough indication of performance: on this dual-core Intel Mac, 2GHz, I can generate 1000 sinewaves in SuperCollider (random freq between 100 and 1000 Hz), at 44.1kHz and with jack's buffer size left at its default of 1024, with a comfortable DSP load of 76% on one of the cores. There are no audio dropouts even if I do some big compiling tasks etc - they naturally get assigned to the other core, of course.)