If you want to be confused even further, attached is my inexpert stab at numeric_limits for NTL quad_float which I think may be the same.
But I have asked the package author for his advice and help on this, but so far no reply (though there are regular package updates).
I'm not sure that epsilon has a exactly defined value but I concluded
static quad_float epsilon() throw() { return 2.4651903288156618919116517665087069677288E-32;}; // 2^(1-106) == 2^105
was my best guess.
Normally it should be std::pow(two, 1-std::numeric_limits<T>::digits), so your value sounds about right (you can also use ldexp(1.0, exponent) to achieve the same thing a little more efficiently). On Darwin numeric_limits<>::epsilon returns something like 10^-303 which is the same as the value for numeric_limts<>::min() I believe. It's just plain crazy, all IMO of course :-) John.