Since this question has already been bumped back to the front page, I might as well add some new information.
In the iOS 7 preview, Apple demoed a feature called Activation Lock. From the iOS 7 features page:
New security features in iOS 7 make it harder for anyone who’s not you to use or sell your device. Now turning off Find My iPhone or erasing your device requires your Apple ID and password. Find My iPhone can also continue to display a custom message, even after your device is erased. And your Apple ID and password are required before anyone can reactivate it. Which means your iPhone is still your iPhone. No matter where it is.
(Emphasis mine) If you remotely wiped a stolen device (to avoid loss of private data), the phone would prompt a thief to enter your Apple ID and password before it would reactivate. If they don’t know them, then the device is effectively bricked.
iOS 7 is still in developer preview and features are subject to change, but since that was mentioned in the WWDC keynote, I’d guess it’s here to stay. It also means that nobody has given Activation Lock a thorough testing, to see how easily it can be bypassed (or they have, but haven’t spoken about it because of the NDA). I would guess it will probably be beyond your common thief, thus providing an effective bricking mechanism.