Since yesterday appengine is also available via IPv6, but the iPhone Simulator does not handle IPv6 all that well. So if there is IPv6 available on the Mac running the simulator, and the app connects to appengine requests seem to fail at random right now, not returning any data. In my case I had tunnelbroker providing me with IPv6 and request from the Simulator were not getting through from time to time, so for now I decided to shut down the tunnel. Just a quick info for everybody who might face a similar, random seeming request fail.
Code from the sideline 