The actual cause is unimportant. A handheld recorder should be able to withstand being handheld in a sweaty hand, and exposure to normal atmospheric conditions that humans live in. The exterior surface should not be expected to be the weakest link upon exposure to normal humidity, sweat, air, etc. in the environments we live and work in.
Hey industrial designers! Stop using this crappy rubber coating stuff! Your valuation of immediate hands-on tactile feel over function and is at best short-sighted for your product's longevity, and at worst planned obsolesence.
The two original R-09s with no rubber coating that I bought 6 years ago are still looking good and working fine(the input jacks on the early ones were weak, but the battery door which worried everyone far more at first never broke on any of them that I've heard about). The little Turtle Beach Audio Advantage USB soundcard I bought at the same time had it's tactile rubber coating go horribly sticky over the entire surface in 3 years. It was in an airconditioned house, hardly ever handled. I speculate normal quanities of ozone in the air attacks the rubber, just like old car tires.
I never upgraded my R-09s to the HR version, and the rubber coating on them bothered me from the start. I knew it would only be time before it went sticky.
Designers, please. Don't use it on anything that isn't disposable!!