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I've bought a butternut neck blank from Tom Thiel of Northwind Tonewoods. When I got it, Tom told me I would need to add some sort of reinforcement -- steel, aluminum, or carbon fiber rods -- because butternut has low compression strength and would compress (and therefore warp) from the load of the strings over time. Tom suggested two 1/8" x 1/2" carbon fiber rods embedded in the neck, on each side of the truss rod.

Casting about for more information, I came across this gorgeous guitar by Bruce Petros: http://petrosguitars.com/gallery/Petros ... el_13.html
It has a butternut neck. I wrote to Bruce, and he replied that he uses two 1/16" x 1/4" carbon fiber rods in his necks. Quite a lot smaller than Tom's suggestion.

I think I'll go with Tom's suggestion, using the same philosophy as sabotage demolitionists do with the quantity of explosive: better a pound too much than an ounce too little. Is there any reason why excessively large reinforcing rods would be bad?

With all that carbon fiber in there, will the truss rod have any effect?

I used the same thing and laid it out following the neck taper. Worked like a charm.

You can still use the truss rod to set relief with a couple carbon fiber rods.