Once you have the book looking nice in Book Designer, it takes literally 5 minutes to create a "real" Mobi book in Mobi Creator. I've written a tutorial
explaining how to do it.
It's very well worth doing - a Mobi book created with Mobi Creator (or tompe's equivalent PERL toolset) is hugely better than what you get out of BD. BD is great for Sony Reader books, but it's not very good at all for Mobi books.