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Where have the Sun/Oracle Java packages gone?
Up until Ubuntu 11.10 Canonical distributed the Sun/Oracle JDK through the partner repository under the Oracle DLJ agreement that allows individuals and companies to re-distribute Oracle Java.
When Java 7 was released, Oracle officially retired the DLJ which means that no more updates to Oracle Java 6 will be issued under this license agreement (see http://robilad.livejournal.com/90792.html).
The partner archive still contains the sun-java6-* packages for releases prior to Oneiric; however it's likely that at some point in time these will be withdrawn.
You can still use Oracle Java 6 - however you will have to download an install it from http://www.oracle.com/java and it won't come nicely packaged for Debian or Ubuntu.
That said OpenJDK 6 is pretty good these days so if you really need Java 6 why not give it a spin and report any bugs that you find...