I got my iPhone via the same cashback deal. The original ad was a link to explain the terms of the deal. I highly doubt that you will be able to get cashback because a short link advertising the 35% off on an iPhone purchase didn't say "new customers only" if it was clear elsewhere. I read what you wrote - did you ever contact AT&T to cancel your purchase immediately?
With all due respect and I don't know, I've heard of many people wanting that 35% off so much that they kept their iPhones beyond the return time and kept going after Bing to honor it. I always make sure that I get a receipt from cashback in email, which is usually generated right after the transaction. Why didn't you cancel and return the iPhone when you discovered you wouldn't get 35% off?
How should we know? If AT&T said they did what they did and thousands of other people don't have the same problem you are having, my guess is that AT&T did it right. If you want to claim that it didn't show up on your page, then go right ahead and sue Bing in small claims court. After being given the option to refund your money, I doubt anyone will really entertain your claim. Willing something to happen isn't just going to make it happen. There are many other people who tried to do the same thing you did.