Day hiking I don't usually listen to my I-pod, but I'll have it with me in my pack. My I-pod Nano has the radio function, so I can get local channels when car camping at local parks. On longer hikes, a playlist of some heavy stuff; Creed, Collective Soul, or Audioslave can get me pumped for a steep climb, or some easier stuff like Robert Plant & Alison Kraus' Raising Sand CD for a peaceful morning watching the fog lift & the wilderness come to life. Hanging out in the rain can bring out some Blues like; Clapton, Robert Cray, or Eddie Vedder. And of course, there's always a time when some Classic Rock might come in handy to have around.
Yes, nature is music and it's wonderful to see it unfold in front of you while outdoors, but I enjoy taking my music along for the walk.