That's not entirely true. Cloud Foundry (the software) is an open-source platform that allows for compatibility across providers. You can see a list of compatible providers here, in addition to cloudfoundry.com: http://www.cloudfoundry.com/partners
Full disclosure: I work for AppFog, one of the providers listed.
> That's certainly a consideration, but every PAAS is "unique" by definition.
I don't see how this is true.
There is nothing about PaaS that says there can't be compatible competitors. Hell, there have been PHP shared hosting platforms that were very nearly identical between providers for a decade before anyone started saying PaaS, and really, the "Innovation" that made shared hosting PaaS lies in per-usage billing.
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.