sounds plausible to me. i bought a iPhone with one carrier and they provided the unblocking codes (via apple) to allow it to be used with any other carrier. Does the definition of 'factory unlocked' apply to this, not sure, but i believe so.
what do you mean by sim adapter? do you mean the plastic thing that allows you to use the micro sim in a normal phone or iPhone 3?
sound to me that the phone you have is not a 'new' phone and as such possibly jail broken rather than factory unlocked
J