[Galaxy S III General] BarryH_GEG:
This has been said a couple of times and is confusing. The only reason you need a dumb-phone IMEI is to give it to the AT&T CSR when they ask. It serves no other purpose and you don't even need to be in physical possession of a dumb-phone. We already know that AT&T's not reading the IMEI when the phone registers on the network. If they were, they'd detect the moment a dumb-phone SIM was transmitting from a smartphone. So basically as long as the s/n of the SIM you're using is associated to a dumb-phone data plan it shouldn't matter what device it's being used in. The exceptions being any device AT&T's sold and Nexi because they do somehow screen those phone's model numbers from the IMEI.

Here are my experiences...

New Service - E-mailed the Premier rep and set up a new dumb-phone account with voice and the $10 data plan. He overnighted me the SIM. Popped the SIM in to my i9100 and went online and completed the online activation process. Never had a problem. Here's what happened in the background. The credit-card sized holder that the SIM pops out of has a barcode on it. The barcode has the s/n. The rep set up my dumb-phone account and scanned the barcode so the SIM he sent me was associated to my specific plan. At no time was there any mention of an IMEI and my account hasn't shown one for over a year. The SIM is what identifies you and your account to AT&T; not the IMEI of a device you’re using.

Adding a Line - When I got my UK G-Tab I added it to my account as a second dumb-phone so I'd have unlimited data and messaging. I can also use the SIM for calls if I ever want to. E-mailed the Premier rep and told him the changes I wanted to make. He made them and sent me two new SIMs; a micro and standard-size because he didn't know which I needed. A note said e-mail him with last five digits of the one I ended up using so he can associate it to my account. I did, stuck the SIM in my G-Tab, did online activation (because it was a new device on my account), and all was good. Again no mention of an IMEI. So there are two devices in my account and neither have an IMEI associated to them and no dumb-phone has ever actually touched my account.

Standard to Micro SIM - When I got the One X I remembered I had the extra micro SIM from when I added the G-Tab. I e-mailed the Premier rep and said I wanted to change the SIM in one of my devices and told him to change the i9100's SIM serial number to the one I sent him with my request. He e-mailed me back asking if I was sure because the SIM s/n I asked him to change wasn't associated with the i9100's phone number; it was being used on the tablet. I said "duh" and he made the change correctly. Within an hour of the change the new micro SIM started working in the One X. I didn't have to activate it because the account was already activated. Activation is a one-time event tied to your account, not a specific SIM card or device. For example, if you go in to a AT&T store to get a new SIM for any reason, they scan the barcode and from that point forward the new SIM is registered to your account. You don't need to activate it. Same thing for people with real AT&T phones. If they get a replacement phone as a swap for warranty issues it doesn’t need to be activated.

Hope this helps. There are lots of different approaches discussed here so by all means pick the one you're most comfortable with or makes the most sense to you. I certainly don't want to be responsible for screwing up someone's data plan.