The iPhone works fine in the US. I used my old 3G perfectly fine a couple of years ago when Mrs D & I were over there.
Whatever you do, don't use any data. Roaming data is a rip off.
Data roaming should be disabled by default (under Settings > General > Network). To be extra safe, you can also disable Mobile Data (same section) and Location Services (Settings).
If you must use the internet, stick to WiFi. I made use of free WiFi whenever possible, and only had to pay for it in a couple of places.
Texts are free to receive, but you get charged for incoming & outgoing phone calls. Cost depends on tariff.
As Shadow Demon UK said, O2
used to let you use your inclusive SMS allowance to send roaming text messages, with one roaming text message taking up four of your inclusive text messages.
If you still have a fixed number of text messages as part of your O2 tariff then you
should still be able to use them towards sending roaming text messages (but should double check).
If you are on one of the newer tariffs, with "Unlimited SMS", then you cannot use inclusive messages to send roaming messages.
You should also divert all calls to Voicemail, or switch on O2CallAlert, otherwise you'll get charged everytime someone leaves you a voicemail message (as it's effectively the same as receiving a phone call while roaming). See here:
http://www.o2.co.uk/support/generalh...email901abroad