Subject: Why are inertial and gravitational mass equivalent?

I know that general relativity says that it is impossible to tell accelerated 
motion from a gravitational field, and that this implies the equivalence of 
inertial and gravitational mass; but is this a consequence of 
General Relativity, or is it an assumption (the way the constancy of lightspeed 
is an assumption of Special Relativity)?

Re: Why are inertial and gravitational mass equivalent?