For those interested in how to build a sensor node system with a Google Appengine backend, here is the process by which I created it. Of course, you should have the hardware part done first!

1. Listen - designing the parser for the computer that grabs XBee packets, and extracts the useful data
2. Store - how to use GAE to store the data in 'the cloud'
3. Graph - using Google Visualizations to make pretty graphs