Bradley Holt has pointed out the portion of the roadmap for the Google App Engine that includes PHP:

According to the newly posted App Engine Product Roadmap, there are plans to add "support for a new runtime language." [...] Looking at the issue list, support for Java is the most requested item followed by support for PHP, Ruby, then Perl.

Be sure to show your support for the inclusion of PHP into the system by hitting a star rating on this page.