If you are on a mac you can use soundflower to route the audio back to your processing sketch 

Thanks for that little command!
Is it possible to do fft's and the like in the getLineOut() command?

EDIT:
After a quick and dirty little mock-up of code I have this, but when I try to run it on ubuntu 10.04 and amarok nothing happens and in windows it gives a buffer error, can anyone help me?
The error that appears in the console is:
=== Minim Error ===
=== Likely buffer underrun in AudioOutput.

1. import ddf.minim.*;
    
   Minim minim;
   AudioOutput out;
    
   void setup()
   {
     size(760, 140, P3D);
     textFont(loadFont("CourierNewPSMT-12.vlw"));
     textMode(SCREEN);
    
     minim = new Minim(this);
     // this should give us a stereo output with a 1024 sample buffer,
     // a sample rate of 44100 and a bit depth of 16
     out = minim.getLineOut();
   }
    
   void draw()
   {
     background(0);
     stroke(255);
     // draw the waveforms
     for(int i = 0; i < out.bufferSize() - 1; i++)
     {
       line(i, 50 + out.left.get(i)*50, i+1, 50 + out.left.get(i+1)*50);
       line(i, 150 + out.right.get(i)*50, i+1, 150 + out.right.get(i+1)*50);
     }
   
   }
    
   void stop()
   {
     // always close Minim audio classes when you are done with them
     out.close();
     // always stop Minim before exiting.
     minim.stop();
    
     super.stop();
   }

An AudioOutput let's you send audio to your speakers, it does not let you listen in on what your computer is already generating. local_guru is right that the only way to do this is by finding a way to reroute audio to your sketch. The buffer underrun error is a red-herring in this case and something I need to fix because I think it's just been confusing people.

Is there a tutorial or something along those lines that will help me accomplish this?