Thanks Budvar10 for replying.
I've changed the rule as you suggested, added my username to the plugdev group, restarted the udev process, and even rebooted.
Sadly, the problem still remains:
avrdude: Warning: cannot open USB device: Permission denied
avrdude: Error: Could not find USBtiny device (0x1781/0xc9f)
So it's back to the temporary patch for now... 