Please Note: The following is an approach I have put together to help other developers stop this kind of highly immoral piracy. I have not tested this myself as I am not currently maintaining any paid apps. Please report back with your results if you do try this.
Background
This section is rather long-winded, so if you’re impatient and/or an iOS ninja/expert, skip down to the “Rough Solution” section.
Most iOS developers know that jailbroken iPhone/iPad users can quite easily download and install cracked versions of their apps. This egregious violation of copyright has understandably angered devs which have poured their hearts and souls (not to mention countless hours and Benjamins) into these apps. Through this anger, methods have been developed to detect and track these illegitimate users.
But what some developers don’t realise is, there is another method for installing cracked apps on your iPhone without jailbreaking. It is not quite as easy as the jailbroken route, but I found a couple of programs which simplify the process. Google turned up a couple of results, Crappstore (a Xcode project), iReSign (automated tool) and IPA God. Part of the complexity lies in the fact that you need access to a iOS developer account as the method requires a valid Development provisioning profile. This should be a impassable barrier for most users, but there are services selling provisioning profiles to users which include their UDIDs.