Google App Engine: REST, Frameworks, Lock-In and Agency Work

I've been doing a little experimentation with Google App Engine for a minor side project, and in the process of so doing, I've been discovering that App Engine seems reasonably well-suited for delivering a RESTful application.  I'm not ready to talk specifics or open-source the codebase for further examination, but I may get there.

In some ways, App Engine seems lower-level than I'd thus far; simple things like binding a URL to a template can't be done without a bit of code.  It seems like there's room for the framework to get a little thicker to cover the most common cases.  In essence, if enough people were going to use App Engine, I think there'd be some value in an App Engine framework.

Unfortunately, the primary problem is still this one: if you're building a long-term application that you expect to use and maintain, then doing it on someone else's infrastructure over which you have very limited control is a risky strategy -- you're making Google a silent partner in your business.   Google's a better partner than most vendors, I imagine, but it's still a pretty big leap of faith.

I'm curious App Engine would work in an agency environment where you're developing relatively short-term projects, where the infrastructure is rarely under your control, and where tying the ongoing costs to actual usage might make a lot of sense.  It's possible that this kind of approach might even be a differentiator for some agency work.  Anyone doin' it?