I am currently in the United States and purchased the tru-phone sim card for my iphone 3G (online through truphone.com) Received by courier 2 days later. Tested the sim and phone worked straight away, with data and internet working also after I downloaded an installation program from the website (in the words of Tony Abbot, I'm no tech-head). When I touched down in LA, switched phone on and started working straight away. SMS no problem, it actually works like clockwork. I used the sim card today to purchases wi-fi in-flight on Delta Airlines and book a limo for my arrival in Orlando one hour later. Very good! Don't get scammed by Optus or Telstra offering something similar at a truly astronomical cost for MB data allowance. Their data plans are $20 per MB (optus) versus 15c per MB for tru phone. I'm sold! I have also used the phone to message in LA, New York and now Orlando florida with no problems. It's great being offered the american cell phone number as an option online. And i can close this all down when i come back to OZ without any penalty until next time i travel (most likely Europe).
Word of caution for others reading this. Tru only uses T-Mobile in the USA, meaning that yes, you'll get 15c/mb data (Which is far from cheap really, but is cheap for roaming), but unless you have a handset that supports 1700mHz (which no iPhone does), you'll be stuck on 2G GPRS or EDGE data speeds, which are not very fast. It's fine for email and the odd bit of light web browsing, but it's not going to be fast. For all the bagging AT&T get, their network really isn't that bad (Optus and Vodafone in Australia are far far worse!).
Most companies will give you a US number with your Travel SIM Card these days.
Some even give you 3 numbers

