You may be interested in http://arxiv.org/abs/0712.0019" [Broken] recent pre-print. Edit: I thought I should add some more info. The paper I linked to argues that the weak field GR result for a smooth lump of mass is not the same as the Newtonian one. While they agree that the weak-field results agree for point masses, or systems with the mass highly concentrated and surrounded by vaccum (stars, planets, black-holes), they argue that this is not the case for diffuse bodies, such as galaxies and clusters of galaxies, where the gravitating mass is spread out over the whole region of interest. Moderator note: to avoid "hijacking" the previous thread of which this was a part, I've split the discussion of this paper off into another post.