This is (potentially) a much much deeper locking to googles cloud (its not just an OS, its your hardware too !).
Of course, it seems django - like, maybe that is enough to avoid the trap?
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."
"Our Users API allows you to authenticate users with Google Accounts"
from some of the quotes earlier,
"you cannot write to the filesystem - due to distributed nature of the system, you have no idea where the file will end up."
"you cannot open sockets! you can only use the limited API that they provide (URL & mail sending API). Forget about Twisted :("
"no threads (Google says they provide scalability in other ways)"
To me looks very specifically that applications built on this framework can not be transferred without a major to full rewrite, thats very much lock in, it doesnt make it easier for google to buy startups, it makes them easier to no longer need to buy them. your users become their users. your applications become part of the google infrastructure.
This looks like the most dangerous and hopfully damaging thing google have done so far
It doesn't in any way disallow you from using something like openid, but your post seemed to imply that.
and you will see the problem ;)