Move from Samsung to iPhone & keep my number

I currently have a several year old Samsung Flyte phone. I am included with my wife's account under the AT&T family plan. My "line" has no data plan linked to it - her iPhone 4S and her sister's iPhone 3 do. The contract with AT&T for my phone/line expired several months ago. I'm thinking about upgrading to an iPhone 4, but I don't want to give up my existing phone number. And, I understand that the iPhone uses a micro SIM card - different that other manufacturer's phones.

I'm pretty sure that I'll need to add a data plan to my "line" - not a big deal. Ideally, we want to add another line to the wife's plan (without data) and give the Samsung phone to her mom, who lives with us. But, I do not want to loose my phone number. We want my new iPhone to have my existing phone number and the Samsung for her mom to have a new number.

Is this possible?