Some time you need a debugger with ability to  step-by-step execution,
inspection and evaluation, not just watch. I use small shell script to achieve this

#!/bin/sh
mongrel-cluster stop
sleep 5
cd $ROR_HOME
/usr/bin/xterm -e "rdebug -n -- ./script/server -p 4001" &
/usr/bin/xterm -e "rdebug -n -- ./script/server -p 4002" &
/usr/bin/xterm -e "rdebug -n -- ./script/server -p 4003" &

The server will stop execution right at the line debugger if defined? Debugger