What's basically happening is the energy is expended deforming the tire, and it's lost in the form of heat and noise (mostly heat). A wider tire generally deforms a greater mass of tire and higher pressure resists the deformation, so those two factors do influence your rolling resistance. While I haven't ever quantified it, I would swear that I feel far more effect from having my tires at the proper pressure than I do from lowering my head. The second link that Danno posted has a nice graph showing tires under different conditions experiencing significantly less rolling resistance as pressure increases.
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