The other issue is... voluntary filtering won't work as long as there are greedy webmasters who don't want to comply.

I would totally be in favor of voluntary compliance, but I just don't see getting anything close to 100% compliance unless it's mandated, and while it's possible that will "ghettoize" the adult community, it's in my book a mandated site label is certainly preferable to a government-imposed solution such as .xxx or something else that won't work worth a crap and will make somebody rich.