So, after trying the generated .war in Tomcat and confirming that it was working fine, it was time to hit WebSphere land, and guess what!, didn't work, duh!
The default web.xml configuration doesn't play well in WebSphere. The worst part is that there are no error messages, not even a trace in the logs to help.
The default web.xml spat out by warbler reads:
The filter doesn't work in WebSphere, but the servlet does the trick. So I changed the web.xml.erb within the installed gem to produce this:
Note that I've repeated the listener tag, it was intentional. WebSphere will complain if the listener declaration came after the servlet tag, it must be before, duh! again!