A very entertaining and somewhat educational article on IBM Poopheads say LAMP Users Need to "grow up". The physical three tier architecture turns out to be the root of all evil and shared nothing architectures brings simplicity and light.
In the comments Simon Willison makes an insightful comment on why fine grained caching works for personalized pages and proxy's don't:
Great post, but I have to disagree with you on the finely grained caching part. If you look at big LAMP deployments such as Flickr, LiveJournal and Facebook the common technology component that enables them to scale is memcached - a tool for finely grained caching. That's not to say that they aren't doing shared-nothing, it's just that memcached is critical for helping the database layer scale. LiveJournal serves around 50% of its page views "permission controlled" (friends only) so an HTTP proxy on the front end isn't the right solution - but memcached reduces their database hits by 90%.