Installing Java's J2RE (JDK, JSE, whatever they call it) on a Debian 'Unstable' GNU/Linux system is fairly simple once you know how to do it. All you really need is the package 'java-package', and the J2RE release from Sun.com. I used the 1.4.2 release.

What I did was:

   ssmeenk@valor$ ls -la j2re*
   -rw-r--r--  1 ssmeenk ssmeenk 14396068 Nov 14 14:07 j2re-1_4_2_06-linux-i586.bin
   ssmeenk@valor$ fakeroot make-jpkg j2re-1_4_2_06-linux-i586.bin

make-jpkg will ask you a number of questions, none of which are too hard to answer, even if you're not a Debian expert.
You'll then end up with a Debian Package file:

   ssmeenk@valor$ ls -la sun*
   -rw-------  1 ssmeenk ssmeenk 20089292 Nov 14 14:08 sun-j2re1.4_1.4.2+06_i386.deb

The generated sun-j2re1.4 package depends on Debian specific files which *are* available from the Debian main archive by issuing:

   root@valor# apt-get install sun-j2re1.4debian

If you built the 1.5 release of the Sun J2RE, you'll need to change the apt-get install line above to reflect your version. D'uh!
After installing sun-j2re1.4debian, and it's dependencies like 'java-common', you can issue:

   root@valor# dpkg -i sun-j2re1.4_1.4.2+06_i386.deb

It will then install the generated Debian package and all will be good. Next we check if the symlinks from Mozilla's plugin directory to the Java plugins are correct:

   root@valor# ls -l /usr/lib/mozilla*/plugins/libjava*
   lrwxr-xr-x  1 root root 39 Nov 14 14:11 /usr/lib/mozilla-firefox/plugins/libjavaplugin.so -> /etc/alternatives/firefox-javaplugin.so
   lrwxr-xr-x  1 root root 39 Nov 14 14:11 /usr/lib/mozilla/plugins/libjavaplugin.so -> /etc/alternatives/mozilla-javaplugin.so
   root@valor# ls -l /etc/alternatives/*javaplugin*
   lrwxr-xr-x  1 root root 65 Nov 14 14:11 /etc/alternatives/firefox-javaplugin.so -> /usr/lib/j2re1.4-sun/plugin/i386/ns610-gcc32/libjavaplugin_oji.so
   lrwxr-xr-x  1 root root 65 Nov 14 14:11 /etc/alternatives/mozilla-javaplugin.so -> /usr/lib/j2re1.4-sun/plugin/i386/ns610-gcc32/libjavaplugin_oji.so

Somehow something on my system has created these alternatives vor javaplugin, i'm not sure what package it was, but i'll update that later when I found out ;). For now, restart your browser, type 'about:plugins' in the navigation toolbar, and Mozilla should show: Java(TM) Plug-in 1.4.2_NN-bNN is installed!