use unlocked iPhone without SIM card?
First off, I live in the US but travel occasionally to Europe. At home I am not a GSM customer, so am looking to purchase a GSM phone for use overseas. I'm considering buying a used, unlocked iPhone, but am wondering how useful it will be to me when I'm back in the US. I don't plan on switching carriers, but would happily use it as a sort of upgraded iPod Touch (since it would basically be a Touch plus camera and GPS).
I thought I had read somewhere that iPhones need a SIM to operate... It sounds dumb, and I apologize if I'm completely wrong and sound like a total n00b (in fact, I'm hoping that's the case, because I'd love to replace my iPod Touch with a used iPhone). Can anyone confirm or deny whether this is true?