Just looking into this myself. Being somewhat OCD, I can never take a vague answer, and ambiguity annoys me a lot.
There is a lot of conflicting info about:
http://forums.roadbikereview.com/components-wrenching/fyi-grease-carbon-seat-posts-40455.html
http://www.bikeradar.com/forums/viewtopic.php?p=15993952
http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=119994
So really, what the heck do we use as an assembly compound (grease is too general a word iMHO) for aluminum (aluminum) headsets pressed into carbon fibre (fiber) frames?
+ everyone says DO NOT use Carbon Assembly Compound (gritty paste)
+ Santa Cruz say use use grease
+ Easton, Campagnolo, Deda, Bontrager say don’t use grease
Looking into it a bit, I think what I am going to use to press aluminium cups into a carbon frame is Dow Corning 7 Release Compound. This is a silicone grease that is neutral on rubber and plastic, and so should be totally fine on carbon and aluminium. It is very water repellent (takes ages to remove from skin, soap and water not very effective). Its viscoelastic properties suggest that at short flow times (or low temperatures), it acts like an elastic solid, similar to rubber. This seems to be ideal for this application. Luckily, I have a tube of this in my arsenal (used for replacing rubber bushings on old monosynth keyboards).
Still looking into what to use on titanium / aluminium, aluminium / amuminium, and steel / aluminium surfaces!