A review paper worth checking out in Molecular Ecology, Variation within and among species in gene expression: raw material for evolution. The salient bit:
We find: (i) microarray-based measures of gene expression are precise given appropriate experimental design; (ii) there is large inter-individual variation, which is composed of a minor nongenetic component and a large heritable component; (iii) variation among populations and species appears to be affected primarily by neutral drift and stabilizing selection, and to a lesser degree by directional selection; and (iv) neutral evolutionary divergence in gene expression becomes nonlinear with greater divergence times due to functional constraint.
In Darwin's Dangerous Idea Daniel Dennett contended that selection is "substrate neutral." If that is so than the wide open realm of gene regulation and differential expression is going to be a twist on a familiar theme.