I assumed that in the months since the announcement of Google App Engine, that its glaring HTTP client deficiencies would have been resolved. Nope.
Any modern platform needs a robust HTTP client (timeout controls, full method support, custom headers, compression support, authentication support, and redirect handling). Unfortunately, GAE's urlfetch client (which the standard Python HTTP clients all funnel down to) doesn't let you tweak various headers (including Referer). Nor can you customize the connection timeouts. Both of these tweaks are tools of the modern day web services programming trade. Subsequently, I have to cast GAE to the tinker toy pile with the rest of today's high-level web apps. A quick look at the app repository proves this out sadly.
On the other hand, take a look at what's been built on Amazon's AWS. Goes to show what you can do when you have an open platform.
That said, GAE does show promise for hosting simple user facing web applications, or offline data crunching/hosting apps (ala google.com) with quick user facing response times and little reliance on the outside web.