Hmm. That's too bad - from my interpretation of some of the posts, I thought iBooks was pulling Category (genre) from an ePub metadata tag, but it's entirely possible that Calibre is working some black magic to get category reflected properly when the ePub goes through iTunes.
Now I just have to decide if the convenience of email outweighs the knowledge that things are "right" and the way I want them.
Thanks for the reply.