I would bet good money that it isn't random. In fact, I've noticed the behavior mentioned in this article. The tracks that we purchased from iTunes are more frequently played than those ripped from our CDs. My wife bought a bunch of Leonard Cohen (which is great) but although he represents about 2% of our playlist, I hear him about 10-20% of the time. Goodbye Mary Annnnnnne...
It's making me not want to listen to our iPod. I suppose I can sit down and create a playlist, but even then, if I include songs we purchased, I'll probably hear them more.
Does anyone have any more info on this?