One suggestion would be:
provide the open-source program in the usual way (maemo extras, etc.) and then, on request per e-mail plus payment you reply with an encrypted database that the user must place in a given directory.

You send the password used to encrypt/decrypt[*] to the user per e-mail (to the same e-mail as the one from which the request originated).
[*] Note: the database stays encrypted and is only decrypted in-memory. This prevents casual users from copying the database (but technically adept users will -- if so inclined -- dump the unencrypted database from memory to disk).

If the password is uniquely linked to the e-mail (e.g. password = hash(master password concatenated with e-mail), then at least there is a possibility of tracing a user who distributes the database with the associated password (because you can "reverse" the password by "cracking" it easily: your dictionary will be your list of registered e-mails, which, I hate to break it to you: won't be huge .

This would at least act as a deterrent for those who might consider "casually" giving a copy of the database to other users.