I think there are a few instances that are clearly unethical:
Putting files on P2P so that all the world can download them.
Ripping a CD and then selling it or gifting it while keeping use of the file.
I like to think in terms of fair household or family use.
Eg. even itunes allows it's DRM files to be played simultaneously on different computers on the same household wi-fi and to be used on several Ipods.
Taking that cue, I see no ethical problem with my teenage son putting my CD's on his Ipod while they are also on my Ipod.
I have 1 SB3, 1 SB Receiver and a Transporter .
I could play the same song in 3 different rooms.
That isn't so different from someone who has a whole house speaker system fed by 1 CD player.
All of this type of use is ethical to me.
I realize ELK draws the line a bit before this but that seems a bit extreme. Eg. I am about to listen to a song, do I call my kid to make sure he isn't listening to the same song?
Again all this use seems to be supported by the record companies that license Itunes for their DRM music. So I feel comfortable following that example.