Charles Bailey's answer is correct. The exact wording from the C++ standard is (ยง4.7/4): "If the source type is bool, the value false is converted to zero and the value true is converted to one."
Edit: I see he's added the reference as well -- I'll delete this shortly, if I don't get distracted and forget...
Edit2: Then again, it is probably worth noting that while the Boolean values themselves always convert to zero or one, a number of functions (especially from the C standard library) return values that are "basically Boolean", but represented as ints that are normally only required to be zero to indicate false or non-zero to indicate true. For example, the is* functions in <ctype.h> only require zero or non-zero, not necessarily zero or one.
If you cast that to bool, zero will convert to false, and non-zero to true (as you'd expect).