AT&T Plans
Slightly divergent question, sorry: wife and I use ATT—two phones on one account—with the second phone costing another $10 per month (family plan deal). Like a couple of others who've posted here, I'd buy a 4GB iphone for $99 (if one does appear), but probably not the $200+ versions. Does the ATT family plan work for multiple iphones ($70 + $10), or not ($70 + $70)? I suppose I could ask ATT or Apple, but since I'm hanging out here just now... thanks.
If you already have ATT, enter your phones into their website to check for eligibility - your phones need to first be eligible for an upgrade. Once past that, you can get iPhones on your existing plan by adding data (for the original iPhone it's $20 per phone, for 3G it's $30 per phone). If you want to dump your existing plan and switch to the minutes in the iPhone familytalk plans, then it would be $70 for the first 3G iPhone ($60 for a 2G iPhone) , $39 for the next 3G iPhone ($29 for the original iPhone). So the cheapest monthly service you can get 2 phones is $109 for the original iPhones and $129 for 3G iPhones. You can also mix and match (have anywhere from 2 to 5 original and 3G iPhones).
Unfortunately, even if you live in a location which doesn't have 3G coverage yet, if you have a 3G iPhone, you still have to pay that extra $10 (you could always visit a big city and start using their high speed network). As a result, there is still a lively market for original iPhones on Ebay and Craigslist in areas which don't have 3G yet, because over 2 years it saves you $240 per phone in plan rates.
If you want text messaging on 3G, you have to add $5 per line for 200 msgs, $15 per line for unlimited, or you can pay $30 and all familytalk members have unlimited. For the original iPhone you get 200 messages as part of your plan and pay extra for more.
So the difference in cost of the iPhone 3G if there is a cheaper 4GB model ($99 vs $199) is almost negligible in this decision compared to the cost of the service (if you're paying $10/mo for the second phone, you'll be paying $39 or $29 per month for your second iPhone. Multiply that over 2 years and it kind of makes holding out for the $99 Walmart phone not worth it. Believe me, my wife and I use our iPhones all the time and would never go back to a regular phone. It doesn't bother me that we paid $399 for the phones - the amount of use and usefulness getting to the internet from almost anywhere is well worth it.