We took phones with us for the first time last October so we could stay in touch with the kids. As others have mentioned, you have to ask your provider to activate the International Roaming facility - this is just so that the phone will be operational. O2 also provide a service (and I imagine the other providers do too) whereby you pay a monthly fee (£3 from memory) and get a reduced tariff on calls. Once you return you simply cancel the facility - in fact O2 diarised it themselves and cancelled it automatically. The calls are still expensive, however, and you pay both to send <i>and</i> receive calls. The cheapest option is to text, although the service isn't available on all networks in the US and your phone will just pick up networks at random, so you have no control over it.
The phones must be tri-band as the US uses different frequencies to those used in the UK.
We noticed that the different voltages played havoc with the charger, although it did seem to sort itself out eventually some time after we returned home.