I swear I've done this before, but either I am forgetting something or the behavior of iTunes 11 changed:
In the past I've only kept about a dozen or so albums on my local (work) computer, but I have networked storage containing my whole music collection. If I connected to the networked drive and copied albums into iTunes, the albums would appear in iTunes but remain on networked storage. When I left my local network, the networked music would still appear, but there was an icon (red x?) to indicate that it was offline. When I reconnected at home, the icons disappear and I can play the networked music again.
Now in iTunes 11, when I drag music from networked storage to my library, iTunes makes a local copy of it in the iTunes music folder instead of just linking to the networked file. Can anyone comment on what's happening?