AFAIK, you can't change device permissions and expect it to survive a reboot without some help -- e.g. using udev.
Add the following line to /etc/udev/rules.d/50-udev.rules
Code:
KERNEL="nst0", MODE="0666"
After the next reboot, the permissions should be set as you want them.
(I have not tested this. Be sure to backup your config file first, and be prepared to troubleshoot.)