That's certainly a consideration, but every PAAS is "unique" by definition. Something like Google Apps has more lock-in than others. Some like Heroku are attractive because they have features not available elsewhere that would take a lot of resources to reproduce.
We chose heroku and Rails for the reasons you suggest, and even tried to avoid major tie-ins to the platform. That doesn't mean it's trivial to move to another platform.
We chose heroku and Rails for the reasons you suggest, and even tried to avoid major tie-ins to the platform. That doesn't mean it's trivial to move to another platform.