The newest science issue (Science, Nov. 6, 2009) published a review article on testing general relativity in the cosmos (Ferreira & Starkman, Einstein's theory of gravity and the problem of missing mass). This review article highlighted related research of professor Pengjie Zhang at Shanghai astronomical observatory.

Fig.3 of the science review article.
Based from Zhang, et al. 2007, PRL

This review article emphasized one method of testing general relativity (GR) at cosmological scales, proposed by Zhang and his collaborators in the year 2007 (Zhang, et al. 2007, PRL).  In this paper, the authors pointed out that, combining weak lensing and redshift distortion measurements, one can test the Poisson equation at cosmological scales, measure the effective Newton's constant and distinguish between alternative gravity models. This method avoids the problem of galaxy bias and stochasticity, is insensitive to the initial fluctuations of the universe and thus allows for robust test of GR. The estimation showed that, the Chinese LAMOST project will be able to test GR at the scale of the order 100 million light years through this method. The planned stage IV projects,  can further improve the accuracy to 1% level and will either confirm or challenge GR at this level of accuracy. This science review article also cited another paper of Zhang and his collaborator (Jain & Zhang, 2008, PRD), in which they presented comprehensive discussions on observational probes of gravity.