iPhone uses SQLite databases for most of its key applications.  These databases are not protected by username/password, and can be queried using SQLite C/C++ API calls.  The following are the iPhone databases and their Root Folder location (/private/var/mobile/Library/) in the iPhone file system:

• Call History - CallHistory/call_history.db
• Address Book - AddressBok/AddressBook.sqlitedb
• SMS - SMS/sms.db
• Notes - Notes/notes.db
• Voicemail - Voicemail/voicemail.db
  
• Calendar - Calendar/Calendar.sqlitedb

To find out the schema for a SQLite database schema, issue the SQLite command ".schema message". Note that the databases cannot be accessed using standard SDK or API, but through some indirect methods on jailbroken iPhone suggested in here.