considering a purchase

You will need to purchase an iPhone data plan in addition to your current voice plan.

I don't think you can just drop the sim card in and go, but you'd probably be best talking to someone at AT&T about this.

 

You'd have to do some hacking if you want to use your existing SIM. I'll be trying this later this week, as I don't want a new plan.

 

there will be a process and it will go something like this:

1) buy new iphone
2) activate it through itunes ( you can't use your old sim card)
3) itunes will ask you if you are a existing at&t customer so you can transfer your old # to the new iphone
4) I am 90% sure that you will need to select one of the new iphone plans and that you will also need to extend your contract with att&t for 2 years. I would verify that with at&t though.

Thats it your new iphone will be activated with your old # and its off you go.