https://groups.google.com/d/msg/google-appengine/obfGjbIkOTI...
"your app can be slashdotted or tweeted by demi moore -- http://adtmag.com/blogs/watersworks/2010/10/mobile-app-creat... -- or perhaps you may need to build/host something on the scale of both the royal wedding blog and event livestream with traffic numbers that are mindblowing -- http://googleappengine.blogspot.com/2011/05/royal-wedding-be... ... these are the reasons for using App Engine. it was not meant as free/cheap generic app-hosting but to provide a premium service that's difficult to get elsewhere in the market. if you're just after the former, there are plenty of options for you."
My take-away is that GAE is hard to justify unless your usage pattern is unpredictable and spike-y. I'm taking the long weekend to give dotcloud a serious test-drive.