Heroku : Ruby/Rack :: Google AppEngine : Python/WSGI

But there's a huge problem -- AppEngine succeeds at seamless multi-tenant truly-distributed clustered hosting thanks to BigTable. Heroku needs to support standard Rails apps, so Postgres is the best they can do, and it's a huge hole in their offering.

You just can't make Postgres (or Oracle) scale on an ideal horizontal the same way you can distribute IP, DNS, HTTP proxying, HTTP serving, memcache, message queues, or bigtable. You can't expose Postgres as an ideal service that just keeps up with what you throw at it.