The biggest problem I see is that they charge per datastore operation, and last I saw they were defining each entity returned as an operation.

If you were building a social network or twitter clone using the techniques they recommend in this talk: http://www.google.com/events/io/2009/sessions/BuildingScalab...

...you could easily pull back a hundred entities each time you pull someone's feed. It'd cost you a buck for every hundred pageviews or so.