Regarding Draft -08
Require that an entry collection either use the atom:id contained in a posted Atom Entry or generate a new unique id for the member.
I'm sure this could be worded better.
Interop. It is currently unspecified how atom:id should be handled on POST operations. There is an existing implementation that reuses atom:id values for different entries, Given that atom:id's are supposed to be universally unique, this behavior is rather unexpected.
Add to section 8.1:
When an Atom Entry document is POSTed to create a new member resource, the collection MUST either use that entry's atom:id or generate a new, unique atom:id value for the member.