Maybe I'm being daft here, but why don't would-be file-sharers just encrypt files and send their friends passwords along with the links? Neither the provider nor the copyright holders would be able to tell who was sharing what, unless they monitored IM, email etc..

Of course it does seem that anyone can abuse the DMCA with impunity, so strictly speaking this wouldn't stop the "notice and takedown" actions. It would however prevent the automated detection and might increase cost and difficulty for copyright complainers.