Update: Took a crash course in mod_rewrite & got the url mask working,
As a side note, if you are using ispconfig with global php disabled you will need to do the following (i don't know how elegant or secure this is) to get the roundcube login to show up @ mail.anydomain.tld
php_admin_value open_basedir none
however now that mail.anydomain.tld functions to serve the roundcube mail login page through Leo's rewrite conditions, I can not login through roundcube @ mail.anydomain.tld, I get a 404, roundcube log shows...
[03-Jan-2008 14:48:29 -0800] DB Error: DB Error: unknown error Query: 8 ** DELETE FROM session WHERE sess_id='8f66526f0cc6c9310131545da1ed5f76' [nativecode=sqlite_query() [<a href='function.sqlite-query'>function.sqlite-query</a>]: attempt to write a readonly database] in /home/admispconfig/ispconfig/web/roundcubemail/program/include/rcube_db.inc on line 505
UPDATE: Works fine!!!! sorry , there was a rogue .httaccess file in */roundcube! silly me.