How to Upgrade Iphone 3GS to iOS 4.0 and Jailbreak Without Original Sim

I have an iphone 3GS on t-mobile and its on 3.1.2.  I want to update it to 4.0 and keep my unlock and jailbreak. How can i do this?  It is my understanding that the iphone must be activated to jailbreak through jailbreakme.com How can I update the phone and activate it without an official AT&T Sim card that is active?  Should i update through itunes? should i just wait ?

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