In order to fix this kind of startup message :

no JDK found - please set JAVA_HOME

or 

Not starting Tomcat : No Java Development kit found

Just do the following :

edit your /etc/default/tomcat5.5  file

and uncomment the value

#JAVA_HOME=/usr/lib/jvm/java-6-sun

Of course don't forget to restart your tomcat

Thanks for this tutorial