Will I be charged a data plan if I buy a factory unlocked iphone?

I want to buy a factory unlocked iphone 4s off ebay. I currently have a 3g that's unlocked but I don't believe it's factory unlocked. If I put my sim card into this new 4s will AT&T know and add the data plan? I've heard this can happen before but is it just for people who buy factory unlocked phones through Apple or AT&T or is it for all factory unlocked phones?

The purpose of buying a factory unlocked phone is so you can sign up with any compatible carrier on a month to month basis, no long term contract.
There's no way to avoid paying for a data plan.

You will still need to get a SIM card from a carrier and have a data plan activated or the phone will not function.