You can download the data from the data store and import it into another database. Sure you would need to write some code to interact with the new data services, but had you not deployed to app engine in the first place you would have had to write that code anyway. All the other APIs are optional and if you are worried about vendor lock in then implement them yourself (again something which you would have to do if not on app engine).
If you are on App Engine though chances are you will use the other APIs like the Taasks API. If you run your own software stack instead you can move it wherever without rewriting any code. With App Engine you either rewrite your code or use app scale.
You can download the data from the data store and import it into another database. Sure you would need to write some code to interact with the new data services, but had you not deployed to app engine in the first place you would have had to write that code anyway. All the other APIs are optional and if you are worried about vendor lock in then implement them yourself (again something which you would have to do if not on app engine).