Anyway, Sun's JRE 6.0 Update 12 added 64-bit support. I recommend downloading Update 14 build 03 to get the latest and greatest. (as of this writing of course)
I've not tried this in Vista 64 or XP 64bit.
Under Linux, download the 64-bit bin file, chmod it to run(I recommend the /opt directory), and it will extract and install itself. Then you only need to add a symbolic link in your firefox plugin directory to the libnpjp2.so file in the /lib/amd64 subdirectory of the newly installed JRE.
Here is what I did under Ubuntu 8.10:
sudo mv ~/Desktop/jre-6u14-ea-bin-b03-linux-amd64-10_mar_2009.bin ./
sudo chmod 777 jre-6u14-ea-bin-b03-linux-amd64-10_mar_2009.bin
sudo ln -s /opt/jre1.6.0_14/lib/amd64/libnpjp2.so