Original iPhone users have been paying $20/month for unlimited 2G data and 200 text messages per month. Since AT&T is no longer offering new unlimited plans, I assumed that I would not be able to keep the unlimited data option when we upgraded my wife's original iPhone. Imagine my pleasant surprise when I upgraded her phone today and was presented with the option of keeping an unlimited data plan for $30/month.
[This is also an option for 3G and 3GS owners who are grandfathered on the $30 unlimited data plan; however, you cannot add tethering to the $30 plan, only the $25 2GB plan. New iPhone purchases are only offered the $15 or $25 data plans. –Ed.]
As an aside: you still have to choose a text message plan, which is a reminder that -- like the iPad -- AT&T very quickly changed the great data package for the original iPhone for a worse deal when the iPhone 3G came out ($30/month for 3G, but no included text messages).
That said: if you don't use more than 2GB per month, you can save $5/month by using the new $25/month 2GB data plan, or the $15/month 200MB plan if you don't use much data at all. My wife and I don't send or receive many SMS/MMS messages, so I'm dropping both of those from our plans and going to a per-use plan. AT&T's per-use messaging costs are $0.20 for SMS and $0.30 for MMS, so assuming that I send/receive 25 SMS messages per month, I'll be saving money there too.