Note that Dreamhost recently moved from using mod_php to running PHP as a FastCGI server (per website/account).
So I assume PHP no longer has the performance advantage of integrating closely with Apache.
In fact, since Dreamhost now offers Passenger (AKA mod_rails) as a solution for running Rails and Django) PHP might be at a disadvantage. Especially when paired with a large framework (zend,codeigniter,cake, etc.)
Speed in a shared hosting environment. I've read about developers running multiple commercial sites from a single Dreamhost account e.g. http://www.pseudocoder.com/archives/2009/01/04/how-much-mone... . That's not realistic with Java, RoR, or Perl.