Originally Posted by
SCochiller
"
Diameter is not the only factor affecting compatibility. Using a carbon-fiber seatpost in a metal frame, or vice versa, invites problems. Metal should be greased, but carbon fiber should not be. The seatpost clamps of metal frames easily damage carbon-fiber seatposts."
http://sheldonbrown.com/seatpost-sizes.html
I think the reason for Sheldon not recommending carbon in metal frames is this was in the era when it was thought that greasing a carbon seat post was a bad idea. This has since been found to not be true. If it still concerns you, just use the carbon assembly paste. It is similar to grease but also contains a fine abrasive to increase the friction.