If you don't want to risk flagging people as crackers and show an alert for jailbroken devices just make your app rely on an external data source that changes.
For example call a specific data source: i.e. version1.0.2.php then during the next update call version1.0.3.php... most of the cracked apps are done with stupid crackulous which doesnt even require a brain... so this is a very good method to use.
I was worried about false positives, so this is the method I utilize most often.
I just display an "please upgrade or unusable" message for the older version.. that way legitimate users simply upgrade and they can continue to use the app, while others.. the downloaders of the cracked version see a note that says oops sorry.. you didn't buy this.. get a job and then buy it
