Matt Soldo (Product Manager, Heroku Postgres)
It will very likely be on the order of 10^3.
FYI, you can hack this by provisioning a database on one app, then manually setting the DATABASE_URL config var on a second app to match that of the first app.
In fact, on my todo list is to research the other database providers for Heroku to see if any of them offer this capability.
Try: https://addons.heroku.com/justonedb or https://addons.heroku.com/cleardb
The rep from Heroku hadn't added his clarification yet.
I should have been more skeptical. Apologies.
Besides being MySQL under the hood, which has known flaws compared to PostgreSQL, with Amazon RDS you get to be completely worry free. You can also do backups on Amazon's S3 and setting up a replicated slave is literally just a push of a button.
Also, I'd hope they will give a few months warning before flipping the switch completely, similar to how the hoptoad/airbrake switch went.
Funny you should say that, as I was involved in the hotpoad/airbrake transition and am now involved in the Heroku postgres one. The transition will be as smooth as we possibly can, no data will be lost, there will be plenty of time to move your data, and easy ways of doing so.
We are really excited about offering a development and free 9.1 postgres service, a huge step forward from the current 8.3 shared database service, and are looking forward to moving people over to the new instances.