I like @astrofrog's solution here - that nicely avoids the question, and we want that behavior to always work anyway.  Sounds good to you too, @taldcroft  ?

@jehturner - Just a quick point on this: I'd rather we *not* write the language of this API in a way that is influenced by a full WCS - I think a lot of astronomers just want a coordinate API for many tasks (e.g. working with catalogs).  (although if we adopt @astrofrog's solution, it's a moot point because the wording no longer enters in the API here at all)