In Europe, tri-band mobile phones are available over the counter without sim cards and no contract with a service provider. You then can either buy a prepaid card with a sim on it, or hook up with a service provider who will send you a sim and you can buy time from them. I guess in US terms, that means these phones are unlocked.
Now, let’s say you buy one of these phones and then go back to the US. Would it possible to simply put an existing sim from Verizon or T-Mobil, etc. into your EU cell phone and have it work?
By the way, tri-band mobil phones aren’t cheap in the EU, but you can buy a new dual band (GSM only) for as little as $40-$50 (in Denmark, probably cheaper elsewhere) that will work just fine in the EU.