For the just released Ubuntu 10.04 LTS, the sun-java6-jdk packages have been removed from the Multiverse section of the Ubuntu archive. So you cannot install it using the command sudo apt-get install sun-java6-jdk!!!
In order to install sun-java6-jdk on Ubuntu 10.04 LTS, You can configure Ubuntu 10.04 LTS to use Canonical Partner Repository via command-line:
sudo add-apt-repository "deb http://archive.canonical.com/ lucid partner"
Then, Update the source list:
sudo apt-get update
Finally, you can install sun-java6-jdk using the command:
sudo apt-get install sun-java6-jdk
