I think allot of developers would like to see the big three (Python,
PHP, RoR) supported in AppEngine, but considering how the sandboxing
has to occur in regards to feature sets, adding a language could be a
monumental undertaking.
PHP in particular has a massive amount of functionality included out
of the box. Allot of python's power comes from it's libraries, so
making modifications gives developers options in adding abstraction
between their app and the Google specific SDK. Most PHP apps are build
on the core PHP functionality, making abstraction more inconvenient
for developers and more difficult for proper sandboxing.
If anything, PHP would be the most likely second candidate for
AppEngine, as it is the most popular web development language at the
moment. Hordes of developers would be looking to port applications
when it became available.