Hi, the DOI system supports some metadata lookup via HTTP 
content-negotiation.

I found this blog post talking about CrossRef's support:

http://www.crossref.org/CrossTech/2011/04/content_negotiation_for_crossr.html

But I know DataCite supports it to some extent too.

Does anyone know if there's overall registrar-agnostic documentation 
from DOI for this service?

Or, if there's kept-updated documentation from CrossRef and/or DataCite 
on it?

 From that blog post, it says rdf+xml, turtle, and atom+xml should all 
be supported as response formats.

But atom+xml seems to not be supported -- if I try the very example from 
that blog post, I just get a 406 "No Acceptable Resource Available".

I am not sure if this is a bug, or if CrossRef at a later point than 
that blog post decided not to support atom+xml. Anyone know how I'd find 
out, or get more information?

Jonathan