In my past experience (Tomcat 4.x), the Tomcat developers recommended against using the Tomcat built-in web server, saying that it was not reliable and was not recommended for production environments. This was before lighttpd and before nginx - and before varnish, etc.
In these days of Tomcat 6.x et al, it appears that things have changed.
Can anyone enlighten me with detailed recommendations from the Apache Foundation and with their own personal experiences?