i know t-mobile,helio, and other carriers it doesnt matter if u have good or bad credit to get a plan but do you need a good credit score to get a plan with cingular?

Pretty much. I had to pay Verizon a $100 deposit, but they can be as high as $500!

As far as Good Credit, I would say it is more about "established credit". Just showing that you have some sort of credit history and no late payments is generally enough for AT&T. If you want an exact credit score, I would say that you must fall into the Average/Good Category low to mid 600's should pass.