FD,
I'd say extended periods of ketosis is abnormal, so the burden is on Atkins to prove it's safe.
These diets are all competing for your hard earned dollar, I only wish I'd thought of them myself. Quackwatch is always a great read, and they have an article on
low carb diets. But then the article appears to ignore the findings of
Kekwick A, Pawan GLS. Calorie intake in relation to body-weight changes in the obese. Lancet 1956; ii: 155-160.
so it might be considered quackery itself.
Just remember, for balance, to have a slice of cheese with your wine when out bon-vivanting.