Back to Google. I'm not an expert, but from what I found there appear to be at least three options:
- The fakeroot method, as detailed by Gianluca here.
- Get it from the unstable repository, and to do that correctly you really need to set up APT Pinning, as expertly described by Weiqi Gao here.
- Get it from the backports repository, as described here.
In the end, I went with option #3, which worked after a slight modification to the instructions. After adding an entry for the backports repository to /etc/apt/sources.list, I had to run apt-get update before issuing the apt-get install -t etch-backports sun-java6-jdk command.