Another way to look at the problem is that Einstein's gravity has no uncertainty principle built into it, it just depends on the energy density, etc. Normally that wouldn't matter, as the uncertainty in the energy density of macroscopic objects is minute, but it is possible to have high mass but not be macroscopic-- at the Planck scale. So there you have individual particles (virtual, as Nereid was talking about) that have an important gravity. But individual particles are subject to the uncertainty principle, and the uncertainty on the Planck scale is enormous (the more you corral a particle, the more uncertain becomes its energy-- that's the uncertainty that make virtual particles possible). So how would a theory of gravity handle an uncertain energy?

But as I said, I'll bet a hundred other things also break down at that scale, and we'd need experiments to know.