I (SvenDowideit) am looking into adding support for OSX Leopard Server to the TWikiInstallers. So far, I've found out the following:
unlike Tiger, Leopard uses Apache2.2
the apache config files you need to use are in /etc/apache2NOT/etc/httpd
ln -s /Library/WebServer/twiki/twiki_httpd_conf.txt /etc/apache2/sites/twiki.conf and then apachectl restart will get you going (assuming that you've edited the twiki.conf)
even with Leopard server set up to relay email correctly, the sendmail setting doesn't seem to work, presume setting SMTPHOST is more effective.