I'm sure you don't want to think about this at this point in time, however you want to also look into how to make it more secure. If its Obj-C, it can still be disassembled (machonist), decompiled (code-dump), etc, and where/how you store that the user is registered is important too.
I would suggest using the serial/machine number/activation number scheme. Throw in any little programming trick you can, such as writing your registration validation and storing in C, and call it using function pointers instead of just calling it. Also, obfuscate your method names.
Or you could go the framework route, and use or purchase someone's framework whose already thought of and handled these issues. I can't think of the names, but there are some good free and commercial ones out there.