spec 0.3 says:
4.10 "atom:copyright" Element
The "atom:copyright" element is Content construct that conveys
a human-readable copyright statement for the feed.
atom:feed elements MAY contain an atom:copyright element,
but MUST NOT contain more than one.
The atom:copyright element SHOULD NOT be used to convey
machine-readable licensing information.
What happens when two feeds with differing copyrights get aggregated?
What if I want my feed to generally be copyrighted, but then want some
individual entry to be released with less stringent rights?
Would there be any problem with treating <copyright> in the same manner that
<author> is with regard to <feed> vs <entry> and defaults?