First of all, the security of your web environment mostly depends on your webapplications. Make sure those are secure.
Most of the time when your server is being probed for vulnerabilities, it will be done by an automated script which will be using the IP address of your server as the hostname. Give the default website (the website configuration not within a VirtualHost{} block) the IP address as the Hostname and give it an empty page.
Use the garbage logfile to collect misformed HTTP requests. Look at them regularly to see what people/scripts are trying on your webserver.
The BanOnMaxPerIP is nice to reduce the effect of a DoS attack. Of course you never can stop a DoS attack, but you can reduce the load on your server by ignoring the DoS-er.
For the rest, it's try, tweak and audit.
Hiawatha is great. I love it. This will be my preferred server over Apache.
Thanks. And please, spread the word. The only thing Hiawatha is missing is reputation.