For once wikipedia.org didn’t have any explanations on the history or origin on the slang term 420. I had more luck with snopes.com. The show Mythbusters won’t touch it because it doesn’t involve blowing something up.
Somewhat interesting but I only have heard the term over the last year, or I haven’t been paying attention until then. I’m not surprised by all the misinformation spread about it though. The people I knew in college who were just obsessed with weed believed some of the dumbest things: it was the first time I heard the Phil Collins myth and the guy who told me really believed it. They also had the worst tastes in music and clothing, something to point out to kids if you don’t want them to smoke pot.