Interesting that this should come up so soon after I have been in touch
with U.S. Postal Service.  I emailed them with just this specific
request and here is an excerpt of their answer on 12/04/2003.....

  Response (Jennifer) - 12/04/2003 07:29 AM
  Thank you for visiting our website. The qualifications for Media Mail
  are as follows:

  c. Printed music, whether in bound or sheet form.

  e. Sound recordings and guides or scripts prepared solely for use
  with such recordings. Video recordings and player piano rolls are
  classified as sound recordings.

  This information was found in our Domestic Mail Manual, located at
  http://pe.usps.gov/

  Domestic Mail Manual>Eligibilty>E700 Package Services>E713 Media Mail

If anyone would like the complete email that was sent to me, please email
me off-list at shermanguy@aol.com and I will forward it to you.  As a
side note, I personally have had no problems at all with Media Mail, or
piano rolls specifically at my local post office, and it's a very small
post office -- one would generally suspect that they might not be as
up-to-date about regulations but they have no problem.

Christopher Vermillion