Let me know if you find more.
There are a couple of files you need to have in a jail which don't work properly, but without which it won't start. You'll find this out quickly if you removed all the files on the 4.x removal list.
Empty Files (touch): /etc/fstab Soft link to /dev/null: /kernel Programs (hard link to /usr/bin/true): /sbin/mount /sbin/umount /sbin/init
First of all if you removed all the files in the 5.x jail removal list, the rc.d startup items will be in the wrong order, unless you replace them with dummy files:
FreeBSD 5.2.1: rc.d-5.2.1-jail.tar.gz
Other recommended dummy files:
Soft link to /dev/null: /boot/kernel/kernel Programs (hard link to /usr/bin/true): /sbin/init