I have been using Google App Engine as a CDN to serve up images and some core js, as well as using Google Ajax Library to serve up MooTools for about 5 or 6 months now. It has worked out really really well. In our case, we needed to serve our clients with our in house servers running ESRI's ArcGIS server. Splitting the load between Google and our Arc servers cut the load time in half for some cases.