I would have to agree with Jeff and say that the charcoal mellowing process introduces subtle flavoring elements. At least that's what my taste buds have been led to believe from JD ads and tour guides, etc. The power of suggestion may play into my impressions of JD but I'm pretty sure I've detected quite a bit of smokiness in certain bottles of JD before. I've automatically attributed that to the sugar maple charcoal without giving it much thought, but as we all know, barrel char level may account for some of that.
One thing I'm absolutely sure about: Kentucky Bourbon folks don't want JD calling itself "bourbon". And the folks at JD are more than happy to oblige them.