The function of the ECON button is described in the manual, albeit scattered around a bit in the appropriate sections for heating, auxilary heating, etc.
I can't recall if it mentions that the a/c compressor should be occasionally turned on but as Mark V has said, if what I've heard about it doing this automatically is true, there would be no need.
This automatic "looking after" of the a/c isn't mentioned in the manual. This is one of a few "features" of the car which aren't described in the manual. This could be because the authors thought the info was not essential for the typical user or, it could be because such features do not exist and I have been misinformed.
I know the manual is not an easy, cover-to-cover read but I think VW have done a good job at describing every feature of their complex car. To prove my point, there's only a couple of tips in the Tips & Tricks sticky post on this forum which actually aren't mentioned in the manual:
1) my trick about re-wakening the MFD display after it has "timed-out", and
2) someone else's trick about displaying an assortment of info via the climatronic system after a strange key combination.
Finally, for SuperMario.Dave, using ECON to reduce warm-up time in a diesel engined MkV will have the opposite effect. Yes it will reduce engine load but it will also disable the electrical coolant heater which is explicitly there to speed-up engine warm-up time. I've never had a car before which can feed warm air into the cabin within a mile of startup and this is mostly all down to the electrical coolant heater.
Hope that helps everyone. I'm going now for a lie down ;-)
Stuart
http://www.stuartdalby.co.uk/vwgolfmk5