first download:

ARMv6 Linux - Headful (Early Access); EABI, glibc 2.5, Hard Float (VFP), Little Endian

from: http://java.sun.com/javase/downloads/embedded.jsp

Format the internal SD disk as a ext2 filesystem. (you can mount the disk over USB on your linux workstation by rebooting the N800 with the USB socket already connected)

unpack the java release tar on this SD disk, it will appear as /media/mmc2 in the N800.

change the ownership:

chown user -R /media/mmc2/ejre1.6.0_10/
chgrp users -R /media/mmc2/ejre1.6.0_10/