VannaB, I totally agreed with your answer about diet playing the key role. Any fit person has a six-pack, it's just most are hidden by a healthy layer of fat. Anything under about 25 BMI is good enough. Most 6 packs are for people who are a little obsessed with their whole diet consisting of everything under 3 percent fat etc. Total nerds in other words. Genetics, do as stated, play a small role.
I have a very good friend who admittedly has a six pack. I am 3 years older and destroy him in near enough any cardio led sport (i.e running, cycling, swimming, climbing etc)
However, I disagree with gyms.
http://www.theanswerbank.co.uk/Body-and-Soul/Q uestion358379.html
If you read my last answer above. You do not need a gym. Nasty, nasty places with ill-advised and pointless instructors.