An Apple software update is disabling iPhones that have been unlocked by owners who wanted to choose which mobile network to use.Earlier this week Apple said a planned update would leave the device “permanently inoperable”.
http://news.bbc.co.uk/2/hi/technology/7017660.stmAlso see:

Altered iPhones Freeze Up

Since the iPhone hit the market in June, tech-savvy owners of the phone have been busy messing with its insides, figuring out how to add unauthorized software and even “unlock” it for use on networks other than AT&T’s.But the Web was filled Friday with complaints from people who had installed the latest iPhone software update, only to see all the fun little programs they had been adding to their iPhones disappear — or, still worse, see their phones freeze up entirely.
http://nytimes.com/2007/09/29/technology/29iphone.html