So, enabling the PHP extension iconv extension=iconv.so
in php.ini now allows roundcube to load, and I can read messages, however sending is still broken. When I try to send a message, it eventually times out, but without generating error messages in either the roundcube or postfix log files.
The problem is that my roundcube install was working perfectly until I updated to 1.1.0, and any new PHP extensions required for the new version don't seem to be documented anywhere (that I can find).