How to connect external database to AppEngine? Are there any ways (better than using UrlFetch) to connect to any external database from AppEngine?

I already know about hosted SQL databases in App Engine for Business, but they are not free and will be avialable in Q2 2011, so it's not an option right now.

I like datastore very much, but customers don't like vendor lock or have other reasons.
Roughly speaking, no.

The big demand of GAE is the integrated, scalable, cloud backend. Without the backend hardware, it's functionally Django on bigtables. If the customers (quite rightly) don't like vendor lockin, GAE is not for them. 

What features of GAE would you keep, if not for the backend?