Thermally induced packing correlations and apparent microphase spinodal boundaries are studied for conformationally asymmetric diblock copolymer melts using the "polymer reference interaction site model" (PRISM) theory. Microphase instabilities are deduced from predictions of the collective scattering intensities and are correlated with the degree of conformational mismatch, For diblocks interacting through attractive van der Waals forces, but where the Flory mean field chi-parameter is identically zero, the predictions reveal rich dependencies of the apparent microphase spinodal boundaries on the degree of structural asymmetry between blocks which are consistent with recent polyolefin diblock melt experiments. This demonstrates the importance of the coupling between local packing correlations and attractive interactions, and the fundamental inseparability of enthalpic and entropic contributions to the effective chi-parameter governing miscibility. Length-scale-dependent "effective compositions" above and near an apparent microphase spinodal temperature are also calculated. Large enrichments of the local composition are predicted in the highly fluctuating, low temperature, regime which depend on copolymer composition, density, and nonuniversal chain structural features, Marked enhancements of this clustering relative to melt behavior are predicted for concentrated diblock solutions. The present numerical calculations are also compared to previous analytical Gaussian thread model PRISM predictions, and general qualitative consistency is found. Numerical calculations for blends of the same polymers are also performed and demonstrate a very large reduction of concentration fluctuation effects and physical clustering for this macrophase separation case.