It was rumored last week, and Apple has now confirmed it:
'Apple said today that a firmware update to the iPhone due to be released later this week "will likely result" in SIM-unlocked iPhones turning into very expensive bricks...
So what are users of SIM-unlocked iPhones to do? Not run the latest software update, that's for sure. Users can instead pray to the hacking deities — the famed iPhone Dev Team that released the free software unlock, and iPhoneSIMfree, which released a commercial software unlock — to write applications that will undo the unlocks, as it were, if those users want to run the latest iPhone software.'