Hello! Are you sure you are using the BlackBerry Storm 9530 (the CDMA version from Verizon), not the BlackBerry Storm 9500 (the GSM version sold by Vodafone in Europe)? It is possible to use the BlackBerry Storm 9500 on AT&T. To have Internet working, you need to ask AT&T for your TCP/IP setting (including the APN, username and password). This setting may differ depending on your type of contract. They will give it to you and you do not need to mention that you are going to use it on a BlackBerry. Once you have obtained the information, on your BlackBerry go to Options -> Advanced Options -> TCP/IP and enter the correct setting. To have the browser working, please visit http://bb.anworm.com/Home/english, and follow the instructions there to download and install the modified service book. Afterwards you will be able to access Internet from your phone and use most third party applications and the Facebook, MySpace, Flickr applications from RIM. To use other RIM applications and one or two third party applications that require BIS you will have to have an active BlackBerry data plan.
You can also consider to buy the cheapest BlackBerry handset, even a refurbished one, from AT&T on contract. Put the new SIM card into your BlackBerry Storm. You can port your old number over. In the end sell the cheap BlackBerry handset you get from the contract on eBay.