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Dan C. Rinnert801 days ago
Check this out:
http://weburbanist.com/2010/03/12/making-it-rain-eco-friendly-ads-disappear-like-magic/
Pretty neat. The ad only displays when it's wet and disappears again when it dries out. Lasts about 8 weeks. Nicely targeted too!
I guess the downside is if there isn't much rain during your advertising period.