I am currently a AT&T/Cingular customer with a 3G sim card. I read somewhere I just have to activate the iphone and then I can use my AT&T sim card without unlocking the phone... Is this true?

Can I use iBricker or any jailbreak program to do this?

Will I be able to apply the new apple update if the phone is jailbroke and not unlocked?

Will I be able to use my current data plan?

I don't want a new contract and I like my existing plan.

Thanks.

yes you only have to activate
yes you can use apptap installer or ibricker to jailbreak
you should restore your phone to 1.02 before you update to 1.1.1
do this by holding shift when you click restore and you can pick the 1.02 restore image