I recently downloaded the 3.1.3 firmware for my 3GS because I wanted to have a copy of it on my mac. I chose not to upgrade to it, and never to ask me again. So I go to use the washroom, and when I came back, my iPhone was 1/3 updated. iTunes automatically began updating my iPhone without my consent. I managed to get back to 3.1.2 with Saurik's ECID server thing, and it's working now on 3.1.2. But yeah, just be very careful if you have a copy of the 3.1.3 firmware.