Will a Iphone 3Gs Lock-Up and Require Contract if Restored in Itunes?

[Zummrox]

I have an Iphone 3Gs, Jailbroken and Unlocked, running on 3.1. Originally locked to at&t. Currently being used on Rogers with a prepaid sim card with pay as you go. I want the new 4.0 update, so im thinking of restoring in itunes to 4.0, and then jailbreaking and unlocking so i can continue to use Rogers (Jailbreaking and Unlocking is now easy to do on 4.0+). But the thing im worried about is if i restore in itunes, the iphone may lock up afterwards and be stuck on the "connect to itunes" screen, requiring a new contract with at&t to be made in order to gain access to the phone. I live in canada, so this will basically make the phone useless. I have owned iphones before where if i restored in itunes, the phone will stay activated and avalible to use (like an ipod touch). Other iphones i have owned locked up after restoring. I dont know what to do. People mentioned things about "phonebook sim cards" that activate an iphone, no matter what service. Anyone with some info would be a great help. Thanks.