In $TOMCAT_HOME (something like /usr/local/jakarta-tomcat-4.1.29 or something like that) there will be a directory named "work". Depending on your configuration, I can't tell you exactly where but do a:
find work -name guestbook_jsp.java -print
and look at the code that Jasper, the Tomcat JSP compiler built for the JSP. Your NPE should be pretty obvious. I have found that tags like <jsp:getProperty> can sometimes do an NPE when you don't expect it.