Recall that I have added the Debian unstable (sid) repository to my /etc/apt/sources.list in Getting Sun Java 6 On Debian 4.0 With APT Pinning 254 days ago. That is still the foundation of what we are doing today.
The difference between today's command and previously used commands is the -t unstable switch. This switch tells feta to install the package requested and resolve any dependencies in the unstable repository. This switch is necessary because the openjdk-6-* packages have dependencies on later versions of other Debian packages (such as libc) that's available only in the unstable repository.