"Publication is the distribution of copies or phonorecords
of a work to the public by sale or other transfer of ownership,
or by rental, lease, or lending. The offering to distribute
copies or phonorecords to a group of persons for purposes
of further distribution, public performance, or public
display constitutes publication. A public performance or
display of a work does not of itself constitute publication.
Courts have not yet decided if publication occurs by merely posting a work online. However, sale of works online does constitute publication."
From a personal e-mail with a US Copyright Office registration specialist, FWIW. I would have posted it in the chat- but it disappeared. :)