What happens to my phone plan when using an unlocked iphone?

I currently have a unlimited talk/text plan with at&t, I really want an iPhone 4, but my dad doesn't want to upgrade my plan to iPhone, if I were to buy an unlocked iPhone 4, and just put my sim inside, would this change anything on my plan? What I thought is, my plan won't change, I just won't have internet access unless I'm at home on the Wi-Fi internet. But all other features such as talk and text won't change, am I right?

Unlocking a phone just allows you to use whatever sim you want. It doesn't change your plan or get you out of any contracts. If you want to permanently unlock the AT&T iPhone you can do so here: http://gsmunlocklabs.com/unlock-iphone/

Your plan and warranty stay the same. I unlocked my iPhone at: http://www.iphonefort-mobile.com/ and I'm very happy. Using the phone internationally too!