I did post to GAE in an application I wrote about a year ago at the university, but didn't have any special problems. What do you mean exactly by posting to GAE? Posting to a webapp deployed on a publicly accessible GAE? Or behind authentication? Or locally on your development environment??
If you want to access GAE running locally on your development environment, you need to use this IP: "10.0.2.2" which represents your local machine, basically your webapp would then be at http://10.0.2.2:8080 (see my post here for more details about that)
Otherwise, if you have issues with Google Authentication from your mobile Android app to a GAE hosted application, it is always good to take a look at open source apps which do the same and see how they handle it. Here are two which came just to my mind..