To expand on yonel's and Benjie's comments above:
1) Landon Fuller's method relying on encryption check, linked above by yonel, seem to be the only one still not defeated by automated cracking tools. I would not be overly worried about Apple changing the state of the LC_ENCRYPTION_INFO header any time soon. It does seem to have some unpredictable effects on jailbroken iphones (even when the user has purchased a copy...)
At any rate, I would not take any rash action against a user based on that code...
2) To complement Benjie's comment re. obfuscation (an absolute necessity when dealing with any string values in your anti-piracy code): a similar but perhaps even easier way is to always check a salted *hashed* version of the value you are looking for. For example (even though that check is no longer efficient), you would check each MainBundle's key name as md5(keyName + "some secret salt") against the appropriate constant... Rather basic, but sure to defeat any attempt at locating the string.
Of course, this requires you to be able to indirectly query the value you want to compare (for example by going through an array containing it). But this is most often the case.