On PHPMaster.com today there's a new tutorial that talks about running PHP applications on the Google App Engine and makes a suggestion for an extension-based framework (Phalcon) that might work well there...if they allowed you to enable extensions.

While a full discussion of GAE is outside the scope of this article, suffice it to say that GAE is neither a private server nor a virtual private server – it’s a platform. As such, it has some limitations. You cannot simply install extensions in it, nor can you write to disk freely. You cannot alter resource limits on your own and you cannot install custom system tools. It isn’t your average Linux box; it’s a fully managed colossal environment for enterprise-level applications.

He talks some about the transition over from a typical hosting solution to the Google App Engine platform and points out some resources that can help you get started. He also restates the need for extension configuration on the platform, noting that some extensions needed for some common functionality in PHP frameworks (or the framework itself in the case of Phalcon) can't be enabled or used.