Lock-in? Google "obtains no right, title or interest" in what is created; the creator is responsible for what is created and can take it down and host it elsewhere.

From the "Google App Engine Terms of Service" (http://code.google.com/appengine/terms.html):

"6.3. Except as provided in Section 8, Google acknowledges and agrees that it obtains no right, title or interest from you (or your licensors) under these Terms in or to any Content or the Application that you create, submit, post, transmit or display on, or through, the Service, including any intellectual property rights which subsist in that Content and the Application (whether those rights happen to be registered or not, and wherever in the world those rights may exist). Unless you have agreed otherwise in writing with Google, you agree that you are responsible for protecting and enforcing those rights and that Google has no obligation to do so on your behalf."