If they were to invent a CBN strain with very low THC I'd get growing, I'd love to have some green valium I could skin up.
http://en.wikipedia.org/wiki/CannabinoidCBN content increases as THC degrades in storage, and with exposure to light and air.
Someone on grasscity reckons if you leave harvesting 7-10+ days later than normal you'll get a higher ratio of CBN due to degradation.