I've got a tricky SEO problem.
jmount: You can definitely over-think SEO, but the main thing that worries me is re-directs.  In my experience the search engines don't like them much and react differently depending on what re-direct code you return (temporary/permanent).  I would suggest some capsule or extraction of your sub-domain site as your home page and then linking to your sub-domain.  This may not be a complete solution as you now have a landing page and some duplicated content.