Blue Collar MTB says don't grease the post. Lennard Zinn gets word from the experts, who all say "No grease." But then carbon-fiber expert Craig Calfee responds: "I don't know where the myth started, but carbon composites are not affected by grease. Our advice is simple: If the seatpost fits tight, grease it. If it slips, de-grease it. As has been known for many years, when aluminum and carbon fiber contact each other, galvanic corrosion can start."
Craig Calfee is correct that galvanic corrosion can occur between carbon fiber and metal. Putting dissimilar electric conductors in contact -- like a carbon fiber seatpost in an aluminum tube or collar -- can result in corrosion at the contact surface as metallic ions migrate from one material the other to balance the electrical potential. The result is a seatpost that is chemically welded to the seat tube.