[math-fun] credit card numbers

I learned (through entirely licit means) that a friend of mine who lives not far from me has the same first eight digits on her Visa card as mine. (She's probably the only person whose credit card number I know, other than myself and my wife.) How unlikely is this? Does anyone know much about the schemes used in devising credit card numbers? I know that there is at least one check- digit, so the naive estimate 1/10^8 needs to be replaced by 1/10^7 or 1/10^6. Is there any region-coding involved? That would further raise the probability. Jim Propp