How difficult is it to migrate away from Google App Engine?

Question

I am thinking of making an (initially) small Web Application, which would eventually have a potential to grow. All things considered Google App Engine seems like a very attractive option. Say, user base and complexity grows and for one or other reason I needed to leave GAE behind. How difficult would it be to migrate away?

1) Does GAE provide a way to export the database? What format would it be? Would it be difficult to put it under MySQL (or similar)?

2) In which areas (ex. database access, others?) would I have to use GAE API? I.e. which parts of implementation would have to be abstracted away / interfaced?

Edit: 3) Alternatively, is it even worth to abstract away GAE API?