Still Works!
Okay, I can confirm this still works as of Monday, Sept 6, 2010.
Had some problems though (of my own doing really).
I tried following the instructions step by step, but the "Switch to a $5 messaging plan" threw me off and I ended up doing that in combination with the $10 unlimited data for non-smart phones plan. DO NOT DO THIS. It really messed the whole process up.
I got the first round of text messages telling me they had detected I was using an iPhone about 2 and a half hours after changing to the non-smartphone plan. Too bad for me though, as it put me on the 2gb iPhone plan again because I did not have the $30 bundle.
Took some figuring out, but basically I had to "clean" my account up by calling AT&T and telling them I was back to using my iPhone. They asked for my iPhone IMEI, and verified that all my data/messaging plans were correct. After hanging up with them I called back and told them some story about how I thought my iPhone was working, but I have to send it back to Apple as I am still having problems. I then explained that while I was waiting for my iPhone to be repaired I would like to set up my non-smartphone with the $30 unlimited data/messaging/email bundle.
SIDE NOTE: Before calling AT&T back I went online to OLAM (online account maintenance) and made sure I was not set up under the family unlimited messaging plan. This is what makes it hard for the rep to understand you want the $30 non-smartphone bundle, because to the rep, the $10 unlimited data plan plus the $20 unlimited messaging plan makes more sense when you have it set up that way. So you have to make it clear that you want ALL OTHER phones on your account to have a limited messaging plan and that YOU want the $30 unlimited plan for non-smartphones. This worked!
Amazingly, once I did it right, I noticed immediately on OLAM that there were no longer any iPhone plans associated with my line whatsoever. Previously when I was switched to the $10 unlimited data + $20 unlimited messaging plan, it still showed my iPhone data plans as an option.
Once I had the $30 plan firmly in place, i made some test phone calls from my non-smartphone handset (just to make sure it was firmly in the system), then I switched my sim card back to the iPhone. I received the confirmation texts stating I had been switched to an iPhone plan after only 30 minutes!
I went back to OLAM and VIOLA! I have unlimited iPhone data once again.
Just for the record, I switched to the 2gb iPhone + Tethering plan back when it was first offer (June 7th I think). I have since jailbroken my phone, and no longer need AT&T's official tethering option.
And PS, to all the haters, AT&T charging you extra to use the SAME 2gb of data for tethering, is criminal. AT&T restored my unlimited iPhone plan without my requesting it. All i did was put my sim card back in my iPhone, their system did the rest.