Ah - this is because th half you have modelled is subdivided and then mirrored. The open edge along the seam is subdivided in a way that does not preserve the edge.
I agree that it would be good for open edges in a subdivision to only be subdivided in a way that would preserve the plane of the edge. 3DStudioMAX does this (and GMAX), and it makes working with subdivisions much nicer.
Might be a bit tricky to code, but I think it would be a real improvement.
Andy?
-- Jeff
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