On the PHPFog blog there's a recent post explaining their service in a bit more detail and how it provides the PHP applications out there with a huge amount of scalability that a traditional virtual server can't.

PHP Fog is a new type of hosting provider for PHP applications developers to build applications the good old-fashioned way but with easy scaling, reliability, speed, and easy deployment/management compared to traditional shared/dedicated hosting.

They look at a few situations where scalability can play a key role - traffic surge, growth leads to performance degradation and underutilized servers - and how the answers to these issues are best described as "scaling up", "scaling out" or scaling down", all things that more traditional hosting doesn't do well. Cloud-based platforms, like PHPFog (or Orchestra) can give you that high availability you need, so no matter the situation.