If you are from outside the USA and you have a GSM phone then your mobile (or cell phone as we call it here) will work in the USA provided you have a 3 or 4 frequencies phone. For a (much too detailed) explanation see Wikipedia. In the US (same thing in Canada) we use the 850 and 1900 frequencies. In Europe they use 900 and 1800 frequencies. Most modern GSM phones have 3 or 4 frequencies. If you are not sure then google for your phone type and see what it says. If you are planning to go travelling in the USA then please realize that this is a big country and that only the populated areas have cell phone coverage.
Also and this is important: even though you can use your mobile phone here in the USA please be aware that roaming charges can be very very high!
For Canadians: Rogers is offering a travel special so that you can get a bunch of pre-paid minutes for use in the USA for a cheaper rate.
Another possibility is that you look into getting a so called global SIM (provided you have an unlocked phone). There are a few options:
With TruPhone local you can receive calls on your new USA number and make calls (to USA, Canada or Europe) for 15ct/minute. For more information see https://localanywhere.truphone.com/Default.aspx
USA sims http://www.usasims.com/SIMs.html has several optionsÂ
Of course when you are in reach of one of the many free WiFi stations (most tourist hotels have them free of charge) then you can use Skype to make calls back home.