Could someone please cite the source for clearance...I've looked in the 2006 code book, I can't find it. Nor can I find it in manufacturer installation instructions.
Without looking into the codes, I can tell you that James Hardie products requires a 1-2" min. clearance under 4,000 feet, and a min. 2" clearance above 4,000 feet (Western US instructions).
I can't tell you where else I have seen the requirements over the years. Hardie Plank is so often used around here I pretty much know their instructions by heart.