I voted for a more complicated meta tag.
Seeing as the web is a world-wide phenomena I think it's important we not single out what could be considered "adult" based on the values of just one society/culture. A tier-leveled rating system would be essential in allowing different communities to determine exactly what is safe and for whom.
As for having a third party verification system? If there's anything I've learned from reading all those sci-fi novels on my shelves it's this: Don't let any one organization have absolute power and control! EVER!!!
