I'll briefly suggest a Palm TX. No keyboard, but built-in full file system encryption (AES) that auto-encrypts based on a timeout or power off. Set it to one minute, and with one minute of inactivity the device re-encrypts the entire file system. This would be a more than adequate level of security for this situation and does not require much in the way of diligence by the user aside from protection of the actual password.

Yes, I'd much prefer to use the Nokia for its better screen and keyboard, but hard to beat the automatic security on the Palm.