This issue comes up repeatedly on the cocoa-dev mailing list and the consensus answer is always do the simplest thing possible. Determined hackers will break all but the most over-engineered solution. And they’re unlikely to pay for the software anyways. Go for the 80/20 solution: the easy solution that gets 80% effect for 20% effort. In this case, putting something in ~/Library/Application Support/your.app.com/. You might name the file something innocent if you want to obfuscate things just a bit. Using the user defaults is easy too.
Whatever you do, don’t use the MAC address or an other hardware ID. Users with a network home directory (e.g. in a shared lab setting) will hate you. Using hardware IDs is just evil.
If someone is in love with your program so much that they’re willing to break your trial limits, let them. The free software costs you nothing and their good will (and maybe recommendation to others) is worth a lot.
Finally, write software that people want to use and price it for its value. If your price is a good value and people want to use it, most people will pay for it.