Realitivy has the same inherent weakness in it as quantum mechanics. From what I have read, they work well for their respective scales, but run into problems when they broach into the opposite side of the spectrum. If I remember correctly, gravity at the quantum level seems to be a major stumbling block. My friend at UCSD was telling me something about gravity doing very strange things at that level.
I am really fascinated on both subjects, but am still early in my reseach into both theories.