Turns out that wasn't the problem; the 20%-off coupon specifically excludes whole categories, one of which is all MP3 players (and another of which is digital cameras, FYI). The only way I learned this is by entering the coupon through "My Account"and then looking at its "details" from there (i.e., not the generic details link in my cart).

Not a total loss: I picked up a Palm Tungsten T3 for $319.