This is what I basically did.
My brother sent me his original iPhone 2G (4GB), received it in the mail. I first tried it on my cousins 3GS since I don't have to cut any sim cards. I swapped out the sim of the 3GS and popped it into the 2G. I restored it via itunes, and then it came up as a new activation through iTunes. I did the steps with the unlimited data+200txt. The activation finished and ATT OLAM to show up on my cousins account as Unlimited 2G data. I then popped the sim card out and put it back into the 3GS. Took about 15min for it to received a text saying it will be placed into the "correct" data plan. After that I checked OLAM and it had the unlimited 3G data. WIN!
My i4 was a bit tricky. I took out my micro sim, and taped it into 2G tray. I activated it through itunes, with the unlimited data+200 text. it finished and my 2G was functional. I then checked my OLAM and nothing it still showed up as 2GB Pro Data. I waited an hour with the micro sim inside the 2G. and still nothing OLAM showed up as 2GB Pro Data. So I searched macrumors for other solutions and found that maybe a new sim will work. So I did that.
Went to my local ATT and told them my i4 got wet and I sent it for repairs so I needed a sim for my 2G, I even showed it to them. They said no prob, and gave me a regular sim. Went home, popped it into 2G and went through the activation again. Finished and nothing my 2G said no service for about 1hr. I became impatient and called ATT telling them my iphone 4 was wet and it broke and I wanted to activate my 2G with a new sim that I had gotten. Told them about the no service up on the status bar. Basically I was on hold for about 30min, I gave them the IMEI and ICD number, and they gave me unlimited data again. It was a bit funny because the CSR knew what I was doing and she kind of laughed saying she's read about it on the internet. I just said nope I need it because the i4 broke lol
So my 2G received service again, and told me if I "buy" an i4 to call instead of activating it on my own or taking it to a local ATT. So I waited for 1hr just because and called back saying I bought an i4 and wanted to activate it, they suggested I go get a new microsim, so I did. Called back, same thing put me on hold for about 5 min and told me I was grandfathered into the 3G Unlimited for my i4. so yep
DDDD AWESOME
Both CSR were incredibly nice to me, ATT CSR's in my experience are a lot helpful and nicer to me than the local ATT reps.
Anyways, good luck to whoever wants to try the data trick. The CSR told me ATT won't remove my unlimited data so yep, and if they did I'll just call back and nag at them ;P
I know this is more of a "cool story bro" thread but I just wanted to explain my method, and that the trick is still in effect.
