Senor, for more on problems of Grand Unification and how it implies a connection between small scale (electroweak and strong) and large scale (gravity) forces, please read here. Basically the attempt made to combine metric (that is your rubber sheet) formalism and the so-called "quantum foam" of virtual particles failed miserably at first glance by a staggering 120 orders of magnitude. Today we do a little bit better with supersymmetry (still speculative), but that means we're only down to a discrepancy of 60 orders of magnitude. This implies that quantum forces (that are the result of exchanged particles) and the curvature of space (General Relativity) are not yet reconciliable. Therefore you have a problem ascribing inertial mass as due to the "dimple" in spacetime since any other force other than gravity acting on your mass does not "translate", if you will, into a metric formalism.