As you might already know the iPhone is itself not that secure that people might think (news is german):
How do you prevent intrusion of your customers data? As a really pragmatic approach, we can just set up additional encryption for our application data:
So if you use for example core data, then on application start you decrypt the database and after termination encrypt it, using the AES class. These are only some basic thoughts and I think that you are better of using higher encryption like blowfish 448bit or even combine it with AES256bit. These encryption is
Your data might live now a little bit securer, but to prevent also code intrusion, you REALLY should strip symbols and obfuscate your code:
and never ever store any secrets or keys in your code!
Also there are some nice reads from Apple you might consider looking at: