So you've got your Telstra/Optus/Vodafone/3/Virgin Mobile iPhone 4 and you want to use it in the US without paying hundreds in global roaming fees? Should be easy, with a prepaid SIM, right? The answer is 'not exactly' -- but there are ways.
There are only two major 3G/GSM carriers in the US -- AT&T and T-Mobile (the others, like Verizon and Sprint, use CDMA/EV-DO, which is not compatible with the iPhone).
AT&T is the Apple-blessed seller of the iPhone, and is the only carrier that officially offers the reduced-size micro-SIM cards that are needed for the iPhone 4.
AT&T doesn't offer a prepaid plan which includes both voice and data usage. However, it does offer a range of "DataConnect Pass" plans which are designed for use with USB modems -- but can also be used in smartphones.
However, that doesn't mean AT&T makes it easy for travellers to get hooked up -- only its company-owned stores seem to know about the prepaid mobile broadband options, called "DataConnect Pass". A franchised kiosk in the Westfield San Francisco Center flat-out denied that AT&T offered prepaid mobile broadband.
You should be able to pick up an AT&T SIM card free of charge from an AT&T store, and then activate it with a DataConnect Pass by going to this web address (you'll need to do it at a WiFi hotspot somewhere -- AT&T won't do it for you in their store.)
You can only use your SIM with DataConnect Pass on the AT&T company-owned network. Although this sounds obvious, in the US, the country is so large that there are many small regional mobile networks that AT&T has domestic roaming agreements with. You can't roam on these networks with an DataConnect Pass -- AT&T's official line is that you need to be in a "dark or medium blue" area of their online coverage map.
Pricing for AT&T DataConnect Pass plans is $US30 for 300MB, expiring in 7 days, or $US50 for 1GB, expiring in 30 days. We would avoid the $US15 for 100MB "daypass" -- it's poor value compared to the $US30 plan, unless you're only going to be in the US literally for one day.
The DataConnect Pass plans also give you unlimited access to AT&T WiFi hotspots, which may be handy, as hotspots can be faster than AT&T's 3G network -- it has attracted a lot of negative commentary for slow data speeds, blamed on the number of iPhone users on the network.
The main catch with AT&T is that they say you can only use their SIM cards with devices that are approved for use on the AT&T network, and they do require your phone or modem's IMEI serial number at the time you activate your SIM card online. A list of approved devices is here. We haven't tested whether a non-approved device would be accepted in the online sign-up -- if you have, please let us know in the comments below.
If you're planning a trip to the US and want to see if your phone will work with a DataConnect Pass, you can buy one on eBay for less than $20 delivered to Australia (just search for AT&T SIM or AT&T Micro SIM), and then begin the signup process on AT&T's website, entering your new AT&T SIM card ICCID and your existing phone or modem's IMEI number. If either are not accepted, the website will tell you -- and you won't have actually had to spend any money (except for the SIM card on eBay).
If you need help while you're there, AT&T's DataConnect Pass tech support line is on +1 888 334 3781 -- just try not to mention to them that you're using a DataConnect Pass with an iPhone
The other option is the T-Mobile network. T-Mobile runs a 3G network, and they do offer prepaid mobile broadband. However, they use a very unusual radio frequency which Australian smartphones (including the iPhone 3G) don't support -- 1700MHz. You can still use their network with an iPhone, though -- it will just run in 2G/EDGE mode, which provides speeds of up to 200Kbit/s in our testing in San Francisco.
The next challenge with T-Mobile is that while they will happily give you a SIM card that will technically work in an iPhone 4 (in 2G EDGE mode only), it won't physically fit, because it'll be a full-size SIM, not a Micro-SIM.
You can get around this by cutting your T-Mobile SIM card down to size in order to use it in your iPhone 4 if you're game, or just buy a $5 MicroSIM cutter from Amazon.com to be delivered to your hotel.
If you decide to go with T-Mobile, don't just visit the shop and sign up for a "prepaid" plan straight away. There's a much better option than prepaid plans that we discovered on a recent trip to the US -- "T-Mobile FlexPay" which allows you to get contract plans without a contract, and pay in advance each month. You don't have to have a credit check and if you don't pay your bill, the service just switches off. On a practical level, it's exactly the same as prepaid, but you get better plans that even offer unlimited internet access. The $US79.99 plan also includes unlimited calling within the USA, which is handy if you do use the SIM card in your iPhone 4.
Also see: Best mobile broadband for Australians visiting the USA (expands on this article).