Also, from the roundcube sieve log:
[15-May-2013 16:30:36 +0100]: S: "IMPLEMENTATION" "Dovecot Pigeonhole"
[15-May-2013 16:30:36 +0100]: S: "SIEVE" "fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy include variables body enotify environment mailbox date ihave"
[15-May-2013 16:30:36 +0100]: S: "NOTIFY" "mailto"
[15-May-2013 16:30:36 +0100]: S: "SASL" "PLAIN PLAIN LOGIN"
[15-May-2013 16:30:36 +0100]: S: "STARTTLS"
[15-May-2013 16:30:36 +0100]: S: "VERSION" "1.0"
[15-May-2013 16:30:36 +0100]: S: OK "Dovecot ready."
[15-May-2013 16:31:49 +0100]: S:
[15-May-2013 16:31:49 +0100]: C: LOGOUT
[15-May-2013 16:31:49 +0100]: S:To me this looks like the plugin is not even attempting to auth?