mime types and xml dialects

Hi,

When returning a mime type of a resource, if that resource is a dialect
of xml then the more general type is text/xml but the more specific type
can be application/rdf+xml which in turn can be even more closely
specified as a subset of RDF.

Currently what I'm doing is returning text/xml for generic xml
documents, and application/rdf+xml for all dialects of rdf; what I'm
considering is the possibility of adding http headers that specify that
there is a range of mime types possible for a resource, and their
likelihood of being the best type for a resource. 

Anyone done anything with that, pointers to discussions, projects and
similar stuff in the past would be helpful.