Engineer from StackMob here (http://stackmob.com). We agree that the need to extend your API on the server side is a huge part of what we are working to achieve in this space. That is why we have always had our "custom code" offerring, allowing you to extend your API in Scala, Java or Clojure. Our partnership with Heroku has also opened up Rails applications to be extensions to your API as well.
As for lock-in we have always said from the beginning that our customers own their data and can take it with them at any time. I think you may find a recent blog post by our co-founder of interest as well: http://www.stackmob.com/2011/11/why-a-paas-for-mobile-develo...
Ownership of data is just one tiny sliver of vendor lock-in with PaaS. The wider issue is how much "stuff" needs to be reimplemented to move to another platform.
Sometimes that's not so bad - if what you have to reimplement is stuff you'd inevitably have to build yourself if you didn't have the PaaS provider, then you're just deferring the cost of it (potentially "forever"). But often you would make different choices if you did not have the choice of the PaaS.
As for lock-in we have always said from the beginning that our customers own their data and can take it with them at any time. I think you may find a recent blog post by our co-founder of interest as well: http://www.stackmob.com/2011/11/why-a-paas-for-mobile-develo...