I would strongly recommend that it be required that an atom:entry that is published with no enclosing atom:feed MUST contain an atom:head element. If this is not the case, the published atom:entries will be essentially anonymous and thus largely useless. Virtually every Atom processor in existence requires elements of atom:head when handling entries. I can imagine very few applications (other than closed ones) that could make use of atom:entries that do not contain atom:head elements and are not themselves enclosed in atom:feeds.
bob wyman