You can build AppEngine applications using the Django python framework (although the supported version is a bit behind the most recent Django release). Where you lose portability (at least right now) is when using GQL/BigTable for persistence. This is Google proprietary database platform. As Hank mentioned this is one of the biggest reasons to actually use AppEngine, but it is also the single largest locking point.
Here are a couple of links to Django support in AppEngine and GQL/BigTable:
- http://code.google.com/appengine/articles/django.html
- http://code.google.com/appengine/docs/python/datastore/gqlreference.html
