There are legions of reasons why I don't recommend Straight Talk, and this is but one of many. There are ways around this, but it's not worth the time nor trouble IMHO when there's better for the money.
I'll save you the trouble of wading through the
superguide, and just cut to the heart of the matter.
You need/want AT&T reception. You need "unlimited" talk and text it sounds like. You were going to have to go on a serious data diet anyway even with the Straight Talk plan due to their soft data caps before throttling. Before doing
anything, I do think you should sit down and get some solid average numbers off your AT&T usage patterns to get a better feel on how much data you need, but you don't need streaming media and there's
offline GPS maps for the iPhone, which doesn't leave
much of a data requirement once eliminated, and one easily supplemented with WiFi and/or FreedomPop if it's an option.
Your options? One of the "unlimited" talk and text packages from either
Airvoice or
H2O Wireless with a data package fit to your needs. Of the two, Airvoice will have the far superior customer support. You'll also have similar terms and conditions as Straight Talk has regarding usage, with a soft cap of around 5000 minutes and 10,000 texts... but that's only a ballpark calculated off of what AT&T bundles their services for to MVNOs, actual "abuse" points may vary from MVNO to MVNO with Straight Talk being the least tolerant. The only thing you'll lose going with either of these two over Straight Talk will be the ability to roam onto T-Mobile's network. Not a huge loss most places, and certainly not given how little you're wanting T-Mo coverage.
Now, if your T-Mobile coverage
is good and you don't mind losing AT&T roaming... you might want to consider
GoSmart Mobile (owned by T-Mo) or
Platinumtel as you'll get more for less. You just need to carrier unlock that phone which should be trivial given you're out of contract and you should do so anyway. You'll possibly lose 3G data access depending on service area given the iPhone doesn't have GSM1700 support, but realistically, EDGE will be plenty fast for everything you need if you axe out any streaming media or GPS services, and even the GPS services don't need
that much.
Outside of desperately needing the ability to roam off-network, those are your best options. Hope this gets you started!