I've posted a couple of times on this, but you don't have to wait till the 29th to get your credit checked. You can go today and get the credit approval, and it will be good for 3 months.
I'm not sure what score you need to get, but some AT&T reps say they're the toughest. YET.... I don't have the greatest credit due to my balance to credit ratio, but I do pay my bills on time, even if its just the minimum. I'm 24 years old, a college student and I got approved for 4 lines. I say thats pretty good considering the reps saying they're harder than other carriers. I've been approved for 5 lines with Sprint, and 4 lines with Verizon in the past, so I think you'll have no problem.
Please note, you should do this only at a corporate AT&T store, not a franchise. Your credit will be stored in their databases and will be accessible at any other corporate branch.