At the latest Google I/O PHP-support for the Google App Engine was announced. During the presentation the speakers mentioned that PHP now runs on 75% of all websites. WOW!
However, when you use Google App Engine with PHP, you have to get to used to some downsides (no mod_rewrite, no curl). But I think the advantages of the powerful Google environment outbalance it. For more information check out this article from the guys at phpclasses.org: