Switching to AT&T and Keeping Original Number

Hi-- I am currently on a family plan with T-mobile. I plan on getting the iPhone with AT&T, but want to keep my current number. I was wondering how I should go about doing this...? Does AT&T automatically do this for me, or do I need to cancel my account with T-mobile (but wouldn't that get rid of my number)? Thanks!

i know during the switch over from cingular to att numbers stayed the same. but, i would first go into a att location and confirm if they can do that befor canceling anything with t mobil.