I think marketers need to be cognizant of the fact that the amount of media people consume is going up while the hours in a day are not. Last month I started (finally) using the Google reader; as a result, I now have access to more targeted blogs. Thanks to better targeting technologies we have less free time to idyllically read email promos from hardcore commercial retailers.
Retailers need to rethink their entire email marketing strategy. This drop in open rates is, without a doubt, a secular trend.