There will always be folks who will try to scam the system. One way you could possible tie accounts together like this would be to stipulate that the plan would be limited to three accounts tied together at the same address. Then they could use registering the Zune hardware as data to show they are at the same local.
This would mean that random people getting together would have to be willing to register their product with at another persons address. The folks who want to take advantage of a system will always find a way to do this.
As for separate play lists most of this could be done on the billing side so it should not be too much of an issue. For accounts already set up you would have to pick one as the parent account and then choose to tie the other accounts to it. The system would then verify the registration information and attach the billing information together.
Certainly this is not a perfect plan but nothing in a prototyping phase ever is