I have been very cautious about all these Web 2.0 hype fuss. GAE, Cloud, Amazon, Azure... some are good, many are bad.
Here, GAE is simply another of the HYPE in the hype cycle of products in the industry.
We don't want hype. We want good and rock-solid stuff. Not fallible hype with a sheep skin and icing and strawberries and sugar and chocolate coatings all over it while it's core is pretty much paper castle.