Strings in an iOS app can be easily extracted with
Taking a few more steps will make your app more secure.
Chris Hulbert has a good post on the topic to obfuscate your secret keys.
Firstly, the technique is only good for secret keys that are not sent over the Internet. For example, secret keys that are used to encrypt a packet, sent over the Internet, then decrypt with the key on the server.
There are a few techniques of various levels:
hex instead of literal strings
obfuscate the hex with some hashing
disable debug sessions
PS: This deters most of the hackers. But nothing is foolproof.