By unknown reasons Sun JDK was moved to partner repository, so to use sun jdk you need to do this steps
sudo add-apt-repository "deb http://archive.canonical.com/ lucid partner"
sudo apt-get update
sudo apt-get upgrade
and then enjoy :
sudo apt-cache search jdk | grep sun
sun-java6-source - Sun Java(TM) Development Kit (JDK) 6 source files
sun-java6-jre - Sun Java(TM) Runtime Environment (JRE) 6 (architecture independent files)
sun-java6-jdk - Sun Java(TM) Development Kit (JDK) 6
sun-java6-javadb - Java(TM) DB, Sun Microsystems' distribution of Apache Derby
sun-java6-demo - Sun Java(TM) Development Kit (JDK) 6 demos and examples
sun-java6-bin - Sun Java(TM) Runtime Environment (JRE) 6 (architecture dependent files)