We just had one of the internal geek fests at my much heralded employer. Eric presented on Google App engine. Very interesting. One observation that I came away on BigTable and transactions through data store api. That is ALL of the entities that you want persisted in transaction in AppEngine must belong to the same Entity Group.
Translate that to Java and DDD -> Aggregate Roots. The root entity could be an aggregate root and should be persisted as a whole. Interesting way to partition data. Have to play with Python and AppEngine.