I thought you can always activate your iphone, if you have ATT sim card, if your phone is locked to ATT.

I am not sure about iPhone 4s, but i recently bought used iPhone 4 and used my micro sim from ATT it worked without a problem.

Why don't you restore your iPhone from iTunes and try to activate it again with micro sim card you have from ATT? May be it will work.

If your phone is locked to another carrier there is no way to unlock it.

Here is a easy Jailbreak tool if you want to jailbreak:

http://greenpois0n.com/?page_id=18