I followed the OP's instructions and successfully added the Unlimited Plan to my iPhone 4!
The 4 is my first iPhone, and have never had service with AT&T before.
I called AT&T around 3:00PM today (7/19/10) and had them switch my iPhone 4 to the Nokia 2320 (GoPhone). My rep asked only for the IMEI # of the Nokia, and said they did not need the iPhone 4 SIM # because it was already activated, and also that I was calling from it. I asked to add the $30 Unlimited Text, Talk, Web for Non-Smartphones, and the rep said no problem. I hung up and made a call from my iPhone 4, and began waiting for the texts to arrive.
I tried using the internet, but I received the following error message, "Could not activate cellular data network: You are not subscribed to a cellular data service".
I continued being patient, and sure enough 1hr and 40min (4:40PM-ish) later, I received 2 texts, and an email.
1st Text: "Did you know an eligible data plan is required for you iPhone? Call 611 or visit att.com/iPhoneplans for details."
2nd Text: "Your iPhone requires an eligible data plan. We have added an appropriate plan and messaging pkg. Call 611 or go to att.com/iPhoneplans for info"
The email was titled "iPhone Data Usage Alert for..." and stated they had changed my phone to the appropriate iPhone data plan and text package.
I then checked OLAM and I had the $30 Unlimited Plan for iPhone, and the $20 Unlimited Texting plan!
Big thanks OP!
This method works well, you must follow the directions exactly (if you have Unlimited Texting, make sure you remove it before calling the first time. Like the OP said, you can add the $5.00 text plan if you so choose), and most of all, be patient! Good luck!