Public domain sheet music: when is it copyrighted?

Dear Rich: I have a question. I have an online flute consignment shop. I have created a link where I would like to make a piece of sheet music from the public domain available for free download each month. I could go to the local university Fine Arts library and copy music out of old books (which will look like scans of old music), or I could download the same piece of music from a free online source that is already in existence and have it be clean and pretty because someone took the time to typeset it and make it nice to read (I cannot possibly typeset this music myself... for many reasons). But I do not know if this is either legal or ethical. In some cases, there are footnotes that say something akin to, "Not to be used for commercial works." In other cases, there is no such notation. Can I take the music from one of these sites and give it away as a download on mine? The short answer to your question is that you can legally use public domain music provided that the musical notation you mention is obvious, routine, dictated by musical convention, and does not involve any major changes or new arrangements. In other words, if the notation is simply what's typically required to write the music, it's not protected by copyright.  Wait... was that the short answer?
Okay, just in case you need some backup, the musical division of  the Dear Rich staff consulted with Stephen Fishman, an expert on the public domain. According to Steve, it doesn't matter if it takes great skill or musical training to create the musical notation of public domain sheet music; nor does it matter if the end result is digitized. The work may only be protected by copyright if the sheet music contains substantial additional music, is an abridgment, or involves making a new arrangement -- for example, creating a suite for several instruments with harmonizations not previously associated with the public domain work.
How then can a music publisher claim copyright in public domain sheet music? According to Fishman, many claims for copyright in public domain music are improper and based on the fact that music publishers have a strong economic incentive to convince the public that its music is copyrighted (even when it is not). Of course, this information won't prevent you from being sued; it just means you're likely to prevail if there is a lawsuit. In addition, it may sometimes be difficult to separate the public domain version from a popular derivative version -- as in the case of the folk song "Tom Dooley". (By the way, the rules described here may not be the same outside the U.S.)
As for your question about whether the copying is ethical, the Dear Rich staff is unable to comment as they are trained only to wrestle with legal issues. However, you may find some advice here.