I\'m in the planning phase of an iPhone application and am considering using Google App Engine for my server component due to its scalability features.
Push notification
Nope. Google App Engine forbids native socket connections. You either need to use a third party service (like the ones you mentioned) or have another dedicated server for sending notifications.
EDIT: Please note that the limitation of GAE on sockets and outgoing calls isn't a library limitation only. Google servers actually block all outbound network connections; they only allow HTTP requests through URL Fetching servers.
If you like, you can star Issue 1164: Add Support for Apple Push Notification Service.