What happens if I put a regular AT&T SIM card in a iPhone 3GS?

Will they start charging me? Or can I just use the iPhone as a phone on the go and just use Wi-Fi? I already keep it in for 10min and called someone but I don't want them to charge me. Any ideas?

They will only charge if the SIM Card is activated or if you allow them to charge on a pay-as-you-go basis. If neither of those conditions apply, you'll only be able to use Wi-Fi and not cell service.

If you mean put your current SIM card into the iPhone 3GS, AT&T will detect that you are now using a smartphone and charge you for a data plan.

The only way to prevent a data charge is to NOT put a SIM in it, and just use it as an iPod touch like device.

You can't make it your actual phone, or else you will be charged.

Make that your phone