First of all, sorry to hear about your stolen iPhone.
What did you do after your iPhone got stolen? Is your AT&T Wireless account frozen? Are you using different phone? Because your original SIM was stolen with the iPhone, you've got a new sim from AT&T?
Log into your wireless account from your computer and see if you still have iPhone Data Plan on your account. If that's the case, I think you'll be fine.
One thing you want to be careful doing:
I've seen reports here from people sold their 3G/3GS and started to use other 3G smartphone devices that are officially sold by AT&T are forced to switch to regular smartphone data plan which is also $30 unlimited at the moment. Problem with this is that iPhone requires iPhone specific data plan, and unless you have that on your account already, you won't be able to switch to it with unlimited data from other smartphone data plan.
I've been using unlocked phones for a month now and my iPhone Data Plan with unlimited is still intact.
So if you are waiting a bit longer to get your replacement iPhone, do anything to maintain the iPhone specific data plan on your account.