In this new article on the Zend Developer Zone, Wil Sinclair has made a "forecast" of sorts as to the role that PHP could be playing in cloud computing in the future.
There are three kinds of services that are commonly associated with the cloud: Software as a Service (SaaS), Platform as a Service (PaaS) and Infrastructure as a Service (IaaS). Software as a Service has gotten a lot of good press in the past few years, as Salesforce and the like have seen adoption from the smallest shops to the largest corporations.
He points to Platform as a Service as the most likely target for PHP (like the Google App Engine). He also points out some of the other providers for the other types (SaaS, IaaS) like Salesforce, Mosso, Amazon Web Services and Rackspace CloudServers. He digs into a bit more detail on the IaaS services, describing how they work (complete with a fun image) and how your application would fit inside.