Yes, nice and easy.
Update to official 4.0.1 and then JB with comex's site and install ultrasnow thru cydia.
You will need to activate your iphone after you update to 4.0.1 though with an official sim.
I already use ATT but jailbreak and all that so I don't need data plan and apps etc. etc.
If I have some apps on my phone that I didn't pay for, what will happen to them when I official update? Should i unsync my apps, upgrade the fw to 4.0.1, install the necessary apps to get the unpaid for apps running, and then resync the apps?
(Not sure if allowed to ask that question even, if I am not I will remove it immediately)