I remember reading somewhere that Google was trying to get pure Django code to work on App Engine (with limited support for queries, of course), and I am very interested to see how that goes. If that happens, (and App Engine gets SSL and an SLA), App Engine will probably come out on top.

But the competition never hurts.

EDIT: http://code.google.com/appengine/docs/roadmap.html

At the moment, Google only has "bulk datastore import/export tool" on their roadmap, which implies that they are trying to reduce their lock-in at least a little.

EDIT 2: http://code.google.com/appengine/articles/django-nonrel.html

Looks like you technically can run pure Django with some limitations.