Yes, you can keep your old data plan. I went on launch day and got my iphone. I was on a 550 family plan and had told them I wanted to upgrade to the 700 minute iphone family plan as I thought that was the only option for iphones now, and would just pay the extra for SMS.
Well, luckily the AT&T guy screwed up my account and I had to call AT&T a few days later when I realized it because I got a better deal.
I was able to go back to a regular family plan of the 700 minutes for $69.99 (2 lines), plus add the $30 data plan and $30 unlimited SMS for the same $129.99 price as the iReady sheets at AT&T stated. So, now I have the family plan to include SMS for the same price as the iphone family plan without SMS.
So, go to an AT&T store and if they give you any problems, just sign up for whatever they say you need to. When you get home, call AT&T directly and change your plan to whatever you want. They will usually prorate it also to the day of purchase..they did for me