Any multi-piece neck will have plenty of resistance to warps. Having a multi-piece neck means that if each piece of wood is prone to warp in a certain direction - if wood's gunna warp, the way in which it'll do so is predetermined by the physical makeup of that specific piece - then using several pieces means the chance of them all warping in the same direction resulting in an actual warp is pretty low. That said, they can still warp, and as Max said single piece necks won't necessarily warp because of the fingerboard and truss rod. It's also to do with how much moisture is allowed into the wood, if there's a build up on hand goo on the wood or a thick enough finish and the wood's dried out enough pre build it'll probably stay pretty still. The fingerboard may still be some issue though...
More to the point though, the Ibanez RGD range uses titanium reinforcement in two strips down the neck.
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