Over the last few days i've been trying to get an Arduino to to an HTTP Post to a web service hosted on Google App Engine using the enternet.h client. Real challenges there since you can't get an IP Address for an individual App Engine app on the cloud. I did solve the problem by providing the correct headers in the http request so you can post to google.com and their infrastructure will route you to the app engine target on appspot.com. I posted the code here:
would be happy to discuss further.
Ben