Despite all chatter about iTunes and DRMd iPhone ringtones nobody seems to have caught on that Apple is going to make it very, very, hard to share a family music library. Apple is has the same goals as NBC -- eliminate sharing of music and videos among family members. They're just much smarter about it (so much for Jobs anti-DRM message!).
The problem is that an OS X music library belongs to a user account. So do the contacts, phone backup, DRMd ringtones, etc.
So what happens if two people in a family both have an iPhone? They need to sync within their user account. That means they can't share music, because only one account can own the music library.
There are workarounds, but they're awkward, unsupported, and Apple can break them at any time. I've posted a few times about this topic, this 2005 post is the most extensive.