I'll throw down a big vote for looking at Drupal. I run a bunch of mainstream sites on it and it has fantastic capacity to run multiple sites on one installation. There are a lot of features for "throttling" content delivery and the caching system is very good. I'm only just recently re-doing an adult site or three with it so I'm still working out which modules/optimizations/etc. are best for this space, but my experience in the other side of the 'Net is pretty good.
It's a very good system, IMO. I'm not an expert on massively-trafficked sites, but there are many huge sites running on Drupal... The Onion, for instance. (I know that as UW says, high traffic news sites aren't running anything oob, but still, the system is built with stability & scalability in mind.)
Two cents deposited.
