I wish I had known about this before I upgraded to my 3GS.
Saving that extra $15 a month is what kept me from going to a regular 3G in the first place.
If you still have your 2G iPhone, it's not too late. You can go back to that 2G data plan.
Go to AT&T and ask for a new sim and just tell them that you want to activate an old iPhone. The card will be free.
Put the card in the 2G iPhone and connect it to iTunes. You should get the let's get started activating your new iPhone. If you get any other screen, restore the iPhone.
Activate the 2G iPhone and when it asks, you are replacing your current iPhone with this "new" 2G iPhone. Enter your 3GS iPhones phone number when asked and then select whatever 2G data plan that you want. Remember, the plan includes 200 SMS free. Complete the activation and now remove the sim and insert it in your 3G or 3GS iPhone.
3G service will work because it is not data plan specific but dependant on your phone. With AT&T, all you need to get 3G service is a 3G iPhone regardless of data plan. 3G service will even work on a GoPhone data plan.
You now are back to a 2G data plan. This is how I got my plan back to a 2G data plan. I had bought my 3GS on launch and they activated it in store. I then did the 2G activation and have been using the 2G data plan since then.
It works, just do what I did above and you will be fine.