National Amusements just released the expected letter it sent to directors of CBS and Viacom today urging them to consider a merger. The note from Sumner and Shari Redstone, who control National Amusements, says a recombination of the companies that were separated a decade ago would enable them to “respond even more aggressively and effectively to the challenges of the changing entertainment and media landscape.” National Amusements owns 80% of the voting shares at both…