As you might already know the iPhone is itself not that secure that people might think (news is german):

http://www.heise.de/security/meldung/Luecke-in-Datenverschluesselung-des-iPhones-1007818.html

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:

http://stackoverflow.com/questions/2579453/nsdata-aes-class-encryption-decryption-in-cocoa
http://iphonedevelopment.blogspot.com/2009/02/strong-encryption-for-cocoa-cocoa-touch.html
http://pastie.org/974094/

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:

http://stackoverflow.com/questions/2442189/write-secure-cocoa-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:

http://images.apple.com/iphone/business/docs/iPhone_Security_Overview.pdf

http://www.apple.com/iphone/business/integration/