Build mod_security against the wrong set of headers, and Apache 2 will mysteriously begin to segfault in a persistent manner. Check which version you’re running with
dpkg --get-selections | grep apache2.
Seems my shiny new Debian distro running the prefork version of Apache had the threaded (worker) headers installed against it. Duh.
apt-get install apache2-prefork-dev reinstalled the correct prefork headers and Apache is happy again.
Mathiew Dessus has a great article about installing mod_securty on Debian for those interested.