Thanks to everyone who helped me confirm that the GPO PURL server is
down.  An official announcement on the GPO Listserv said:
  "The PURL Server is currently inaccessible. GPO is working with IT
staff to restore service as soon as possible. We regret any
inconvenience caused by the server problems. An updated listserv will
be sent once service is restored."

While the server is down, here is one workaround (thanks to Patricia Duplantis):
  1. Go to http://catalog.gpo.gov/
  2. Click "Advanced Search"
  3. Search for word in "URL/PURL", enter the PURL
  4. Click "Go"
  5. The original URL at the time of cataloging should appear in a 53x note.

This incident, however, illuminates a weakness in PURL systems: access
is broken when the PURL server breaks, even though the documents are
still online at their original URLs.

Maybe someone more familiar with PURL systems can tell me... is there
any way to harvest data from a PURL server, so that a backup/mirror
can be available?

Keith