Using Garage Band sounds for commercial purposes
Actually, you can't. The linked article specifies exactly that you can't:


  to create your own original music
  compositions or audio projects. You
  may broadcast and/or distribute your
  own music compositions or audio
  projects that were created using the
  Audio Content, however, individual
  audio loops may not be commercially or
  otherwise distributed on a standalone
  basis, nor may they be repackaged in
  whole or in part as audio samples,
  sound effects or music beds."


(emphasis mine)

You can create as many songs as you want using all the sounds you have in the loops and sell the songs. But you will be packaging the sounds (or repackaging) as audio samples of your application which seems to be forbidden.