Right, I don't think being interrupted constantly is an awesome thing either, and agree that there are some great technology solutions to cut down on these sorts of interruptions. But I still believe that proximity is a useful feature of open plan layouts.

Having an "open plan" for a smaller team - say under 10 - is probably a good way to go. That way each team can develop rules like this 'Library Rules' idea, or a more interruptive one if thats the way they feel they get more work done.