Heroku's Postgres Dev Plan lets you use PostgreSQL 9.1 instead of 8.x, which is what their shared-database offering uses.

Please note that the dev plan is in BETA, and is not recommended for use in production systems. That said, our experience thus far is that it's quite a bit faster than the previous version.

Also, automatic database backups are not available on the dev plan, so make sure you're backing your data up yourself!