I am familiar with debugging Tomcat in Eclipse, it's a breeze - drop in the plugin, configure and run.
With IDEA, I've tried this howto (http://www.phasesoft.com/articles/tomcat4idea/) and I get the strangest thing.
I can start Tomcat but it says "Apache Tomcat/5.0" and I don't even have 5.0 installed on my system?! None of my apps run but the default Manager and Admin apps. It's probably because I'm running 4.1.27, but I'm not about to revert to Tomcat 4.0.x just so I can debug in IDEA. I've heard that Tomcat 4.1.x is not supported, but since Eclipse's Tomcat plugin works with all version of Tomcat, I figured it'd be simple to do something similar in IDEA. I'm also willing to explore debugging in Resin (3.0.3) or Orion (2.0.2) if they're easier.