Secure way to save encryption keys in iOS

In my iOS4 + application, I use AES encryption in several places, and the whole application should be very secure. To do this, I need to hard-code several keys in this application, which are then randomly selected when I need to encrypt something ...

My question is how to store these private keys? Is it safe to hardcode them with NSString? Or

#define SecretKeyString @"febd9a24d8b65c1c787d50a4ed3619a9"

If the user disconnected the iPhone using this application, could he get these hard-coded keys? How can I hide them most effectively?

Thanks for any suggestion ...