Take a look at Audio Hijack Pro. In my opinion the best app for this ever created
http://www.versiontracker.com/dyn/moreinfo/macosx/18611
And if you don't want to buy this maybe StreamRipper or Wiretap. Just try a search at versiontracker.com for stream.
But AudioHijack is the best for this because it can directly hijack every programms output. You don't have to worry if an other apps audio output "destroys" your sound recording becuase it just hijacks and record the specified app. And you can set timers or DSP effects to make it even sound better