It all depends on what AT&T means by "businesses"....
Fortune 150 companies? I don't see a lot of in-roads here. Blackberries sell because they integrate with Exchange servers (no one really cares if they allow anyone to work with Office formatted files, BTW, that is far too painful). But being able to interface with their corporate e-mail while on the road is invaluable.
Now switch the focus. Imagine a commercial where the Fortune 150 executive looking harried in a power suit is using a Blackberry to stay on top of business while waiting at a gate in the airport. You see him squinting and making small pecking motions at his ridiculously small keyboard in order to "seal the deal". During this time, he gets a call, and after he hangs up, you see him cursing under his breath as he tries to locate his incomplete his e-mail.
Meanwhile some young smartly dressed guy sits down next to him with earbuds in enjoying some song and looking relaxed. He gets a call, pulls out his iPhone and answers it. "Oh hi Jerry! <small talk>. You need more detail on that proposal? Hang on, let me see if Bonnie is available". Switches out and conferences in Bonnie. Bonnie answers some questions and agrees to mail him an altered proposal. "The man cuts in and says go ahead and mail it directly to me, I'll approve it and pass it along.".
While he's still on the original call, he gets the mail from Bonnie, adds his 2 cents, and forwards it along to Jerry. "You should see it in the next couple of minutes.". They talk a little more small talk, setting up a chance to do dinner or something (maybe bouncing out to a web browsing app to find a good place to eat), when Jerry gets his e-mail, says it's exactly what he was looking for, and seals the deal on the phone. When he hangs up, the music resumes.
Meanwhile, power-suit guy finally finishes his e-mail. He calls the person he sent it to, finds out it was accepted, but he may need a few small alterations, and ends the call looking more like he crossed the finished line after a long race, than anything.
The point is, is that the small entrepeneuers may not need Exchange server support or Microsoft Office document support, but they still need to be able to work from the road. The iPhone is the tool for the small business owner that uses tools like Yahoo small business e-mail, but still needs to stay connected.