Tim Bray has posted a proposal on the Atom wiki for a method of tagging an entry as 'not for syndication'. As Tim explains in his entry on ongoing, his motivation is for pictures, and to nix multipart/alternative:

The motivation for doing this was to get rid of the type="multipart/alternative"feature in the current Atom drafts, which strikes me as a "wouldn't-this-be-nice" kind of thing that doesn't do a good job on the most common use-cases. But I think this syndicate="false" idea ends up generally increasing the usability of the whole Atom protocol.

This is good. First I have my doubts about 'multipart/alternative'. Second this broaches the issue of the 'state' of an entry. While 'syndicate' is one such state surely there are others. How about 'draft', which would imply that the content should not be published. And what if you have email subscribers that get notified when a new entry is published, could that also be rolled into the 'state' of an entry?

Now there are two basic ways to go with how to implement this. One is to create a series of 'true' | 'false' valued attributes to an entry, such as draft="true" or email="false". An alternative is to add a single 'state' element to entry that contains different values, for example <state>draft</state>. I have no preference for either solution, but I would like to see something added to track entry state.