I will be visiting USA during this summer. I unlocked my phone so I can insert SIM card of any provider. My phone works on GSM / GPRS 900/1800/1900. I am looking for the cheapest offer to keep in touch with my family in Poland. Can I buy a something like a starer (a new SIM card, with new number) in a supermarket? How to activate this service http://www.att.com/esupport/article.jsp?sid=KB109950#fbid=QUkDAbsvAlE ? I do not want to buy a new phone, and I do not want to sign any contract.
You can obtain a new SIM and fund your prepaid account by visiting a local at&t store once you arrive - imo, that's the easiest way to handle it. AFAIK, at&t does not sell "starter" SIMs in supermarkets, etc. - you'll have to visit an at&t or an authorized retailer. You don't have to purchase a new phone or sign a contract to obtain prepaid service.
That said, if your existing device does not also support the 850 mhz frequency, you may have coverage issues with at&t's service. They use both the 850 and 1900 mhz GSM bands. Plus, at&t no longer offers "Pick Your Plan" prepaid service - only the "Pay as You Go" type is available for new customers.
=^..^= There must be a happy medium somewhere between being totally informed and blissfully unaware.