I don't know the details about the hacked iPhone bricking, but it's not so far fetched that whatever Apple does to update the (hacked) firmware simply happens to brick the device. I don't see why they wouldn't simply revert the device back to the pre-hacked state if they could do it.

And by the way, you CAN upgrade the RAM and even the hard drive in a MacBook without voiding your warranty. I upgraded both in mine. They can be accessed by taking out the battery.