I don't think that article is correct. I believe the Apple stores are only set up to unlock AT&T models sold at full-price. (In theory, they should be able to do this for any SKU, but I don't think they have programmed the point-of-sale system to do so, at least yet.)
I actually have first-hand experience with this, because I went to the closest Apple Store to me in order to buy an unlocked iPhone using the "Device Only" AT&T model trick. It turns out that I walked out the door having paid full-price for a *Verizon* phone, not an AT&T one...they grabbed the wrong model from the stock room! I didn't realize what had happened until I got home and looked at the receipt, which plainly said "Verizon" on it. I know the sales clerk rang it up as "Device Only" because I saw her do it, and I paid full-price, and they never asked me for any account information.
I tried putting my AT&T SIM in it, and it told me I couldn't activate the phone without a valid SIM.
I returned it the next day in exchange for an AT&T one. When I powered that one on, I was given the "Congratulations, you're unlocked" screen on iTunes. So paying full-price for the Verizon model did not work, but paying full-price for the AT&T one did.