Maybe I'm missing something essential, but what's the nature of "support" here? They say I can put any SIM card into the phone, so what's the deal? I'm serious

Maybe I'm missing something essential, but what's the nature of "support" here? They say I can put any SIM card into the phone, so what's the deal? I'm seriously asking, as I'm located in Europe, and wanted to use dev Nexus One with any card I'd put into it.