This article gives you some details on the data protection in iOS. If you have an iPhone 3GS or newer you'll have hardware encryption setup so that if anyone doesn't have the passcode the only data they can see is psuedo-random nonsense characters.
Basically do this to enable hardware encryption:
- Set Require Passcode to Immediately.
- Disable Simple Passcode to allow the use of longer, alphanumeric, passcodes.
- Enable Erase Data to automatically erase the device after ten failed passcode attempts.
The best passwords are a minimum of 12 characters made up of upper-, lower-case, & special characters that are not words.
I am talking about the device police hook up to your phone to extract data without using a passcode. Will the content still be visible or unreadable after they extreact it?
Honestly, if you are at the point the police are electronically snooping around on your device, then there is nothing you can do to keep them from doing it. You cannot completely encrypt the device contents with anything like AES encryption (not sure it could even work - the computation load from having to decrypt anything anytime it was needed would likely make a smart phone into a useless door stop for all practical purposes).