Replying to Jiggy, but can't directly reply since it's dead:
No, a simple Rails app is not very coupled to Heroku.
The deployment process is simpler and pretty unique to Heroku, but it's usually pretty easy to move away from Heroku or deploy in parallel to another location. I actually deploy one app to Heroku and I have an offsite backup process that deploys the same codebase to a NGINX/Passenger setup and clones the production Heroku data to that instance.
Your mileage will vary, of course. If you start using a bunch of Heroku plugins, you'll find it's harder to maintain an easy dual-deploy setup.
> This quickstart will get you going with Python and the Flask web framework on the Cedar stack. For Django apps, please see the Django quickstart.
https://devcenter.heroku.com/articles/python
It can be very cheap or free at low scale,
> Each application receives 750 free dyno hours per month.
https://devcenter.heroku.com/articles/dynos