I discuss several issues that arise when trying to constrain the dark energy equation of state using correlations of the integrated Sachs-Wolfe effect with galaxy counts and lensing of the cosmic microwave background .
These techniques are complementary to others such as galaxy shear surveys , and can use data that will already be obtained from currently planned observations .
In regimes where cosmic variance and shot noise are the dominant sources of error , constraints could be made on the mean equation of state to \pm 0.33 and its first derivative to \pm 1.0 .
Perhaps more interesting is that the determination of dark energy parameters by these types of experiments depends strongly on the presence or absence of perturbations in the dark energy fluid .