感觉不需要迁移到Postgres - Heroku上有几个MySQL Addon提供程序 - http://addons.heroku.com/cleardb 是我取得过最成功的一个。

I'm trying to push a brand new Ruby on Rails app to Heroku. Currently, it sits on MySQL. It looks like Heroku doesn't really support MySQL and so we are considering using PostgreSQL, which they DO support.

How difficult should I expect this to be? What do I need to do to make this happen?

Again, please note that my DB as of right now (both development & production) are completely empty.

Don't feel you have to migrate to Postgres - there are several MySQL Addon providers available on Heroku - http://addons.heroku.com/cleardb is the one I've had the most success with.