I like the idea of having the possibility of running php in a servlet-like engine. I'm thinking of proper control over thread sensitive applications, real-time systems etc.
Saying that one could use Java is true, but not an argument for why PHP couldn't benefit from having such an engine. Where Java is heavy weight and might scare users off, I think PHP should focus on being lightweight, flexible, powerfull and accessible. Just like what PHP makes a success today.
It may also give PHP a new boost in terms of establishing one very strong framework (or framework standard/specification) and improve the enterprise readiness of PHP. That could become a 'plus' over the numerous Java solutions out there (Tomcat, WebLogic, WebSphere etc).
I'd like to pose a new question to this very interesting discussion: how would you see a serphplet being realized? Would this be similar to PHP-GTK? Would this be OO only, or also procedural (begin, end, wait, goto...)?