I have determined that the issue is with .htaccess and permalinks. When I disable permalinks, wp-print works just fine (contrary to what I wrote above... perhaps I was experiencing a cache issue or something?)
The readme that comes bundled with the plugin says to find this line:
RewriteRule ^archives/([0-9]{4})/([0-9]{1,2})/([0-9]{1,2})/([^/]+)/?([0-9]+)?/?$ <BLOG URL>/index.php?year=$1&monthnum=$2&day=$3&name=$4&page=$5 [QSA,L]
and replace it with:
RewriteRule ^archives/([0-9]{4})/([0-9]{1,2})/([0-9]{1,2})/([^/]+)/print/?$ <BLOG URL>/wp-print.php?year=$1&monthnum=$2&day=$3&name=$4 [QSA,L]
Only, that line is not found in my .htaccess file! All I have is this:
Options +SymLinksIfOwnerMatch
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
Is there mod_rewrite expert here who can advise what the proper RewriteRule should be to enable wp-print?