My IMEI number is the generic "someone hacked this phone" one.
There could be a flaw in the update, but it seems unlikely it would do this. This strongly suggests the phone has been tampered with, but not necessarily by you. Has the phone ever been out of your control -- maybe just left on a desk at an office while you stepped away, in a gym locker, etc.?
Consider that there are a lot of cell phone scams out there. I would guess that iPhones are targets precisely because the cost of ownership is high, implying that owners have things worth stealing. Plus, not all the scams have to work -- criminals have nothing to loose if an attempt fails unless they get caught. Something that bricks the phone later on presents essentially no risk of that.
Unpleasant to think about I know, but no computer or cell phone is secure if it is not physically secure.