Easy Encrypted Journal is a fast and free journaling application which securely encrypts each entry with a separate password.
Feeling Paranoid? Easy Encrypted Journal encrypts each journal entry with a separate AES encryption password, so you can be extra certain your journal entries are safe.
Normal journal applications store your entries in plaintext within a database that could be accessible to other programs. Easy Encrypted Journal stores all entries as encrypted cypher hashes that would take supercomputers many years to crack.
You can download it from the Android Market here.
Update 4/2012 This is now open source and hosted on GitHub. Check it out at my GitHub page.