Steve Jobs said that Apple will fight unlocked iPhones a couple of days ago. Apple has done just that with the release of the new iPhone firmware 1.1.1.

The iPhone firmware 1.1.1 gives you the cool wireless iTunes store which is already available on the iPod touch. So there is an incentive to actually install this firmware update. Several sources report though that firmware 1.1.1 is re-locking unlocked iPhones.
Also third party applications stop working after the iPhone update.
The hacking teams around AnySim and iPhoneSimFree are of course working to find ways around the new firmware.
Gizmodo has a good summary of the current state of things.

Now we are totally in the same situation as with the Sony PSP homebrew battle. I am almost certain that at some point there will be another hack or way around the new firmware 1.1.1. What happens though is that it is getting more complex to unlock the iPhone and many 'casual unlockers' will just stop doing it. Since unlocking voids the warranty it is not that wise to unlock your iPhone.

In a perfect world Apple should just sell the iPhone unlocked and let users decide which carrier they want to use the iPhone with. But Apple is aggressively trying to maximize the revenue they can make with the iPhone by participating in the revenue of the carriers who exclusively offer the iPhone around the world.