With respect, Milner I have to disagree. While there is no all-purpose legal definition of publication, once you place a work on the internet which is freely accessible to the  public, then there is little doubt that  whatever the juristic  context it would be regarded as having been published. Editorial policy is of course not constrained by legal niceties. Orbis makes clear that if a work has  previously"appeared on the net" irrespective of how the place it has appeared describes itself , it will be regarded as ineligible. That I suspect will be a common position but not necessarily universal. The statui of work which appears only in the workshop and which is accessible only by members strikes me as  more problematic; however as we know the vast majority of work appears on the submit page. "Submit" here is strictly a misnomer as work is rarely rejected and  even such ill-starred pieces are anyway "published" in the sense described above  before being relegated to oblivion.

In these days of self-publishing, once a writer puts a work "publicly online", it might as well be considered to be published. A writer today must make the call of which avenue to pursue with any given piece of writing: to go online and essentially self-publish, or to go through the controlled channels, be they editor/publisher-controlled online, paper-based or a hybrid.   

I would contend that the very structure of the PC boards signifies that any work submitted to the submit board should be considered by the author to have been published.  PC has a workshop board distinct from the submit/picks boards.  If folks want to be able to claim that they were "workshopping" something, then it should have been placed in the workshop, and not submitted.   Work can reside indefinitely on the PC submit board, effectively in limbo.  Tacit rejection by not being "picked" is indefensible, when there is a "reject board".  Finally, work appearing on the submit/picks boards is returned in Google search results, even with just a few unique words from a poem as the search criteria.  Work on PC submit/picks boards is truly "out there on the net".  It would behoove the writer to consider it to be publishing, when submitting a piece to the submit board (not the workshop).

Even if one does not agree with the above, and might prefer to consider work posted to the PC submit board to be merely submitted, and not published, that fact that a lot of mags/pubs (online or paper) do not accept simultaneous submissions should be taken into consideration.