@Quentin's comment is right - as long as someone has physical acces to your device, it's only a matter of time until it's cracked. However, you can make that amount of time take so long that it's not worth it.
If you were to encrypt your database, the decryption key would also have to be stored on the device (assuming you want it to work offline). You could use the keychain to store this key - then they have to crack the iPhone's keychain before they can get access to your data.
The other alternative is to only let your app work while online - store the key on your server and have the user login and authenticate before you pass the key back to the app. This s a bit more work from you but will ensure that the key and the data are stored in different locations.
Finally (and most securely), you could store everything on your server - that way you control the data nd the key yourself and deice theft won't make the slightest bit of difference. however, if your data set is big this might make the ui of your app more complicated. And it won't work offline, obviously :)