A permanent, universally unique identifier for the entry, in IRI form.
This is null for new entries (i.e. ones which haven't yet been inserted on the server, created with Entry), and a non-empty IRI string for all other entries.
For more information, see the <ulink type="http" url="http://www.atomenabled.org/developers/syndication/atom-format-spec.php#element.id"> Atom specification</ulink>.