You're question #3 raises a red flag. If this is an important issue, I'd caution against App Engine at this time. I love the platform, and don't doubt that their will be viable migration paths to a self-hosted solution at some point, but not now. Things like appdrop prove it would be possible to do, but would the effort and investment be worth it? That's the question I'd ask. I'd love to know if somebody has successfully ported a real-world production app engine app to another host.
Backups should be easily scripted or there are tools like GAEbar as Bolotov mentioned.
Regarding cost, you can probably get tens (maybe hundreds) of thousands of objects (records) and decent traffic/use for free. Beyond that, I'm not sure about comparative hosting costs, sounds like a good area to do some research in (note to self).
Finally, Silfverstrom is right about limitations, especially around full-text search. There are some projects underway to tackle this, but probably nothing as robust as a mature RDBMS.