The scheme is operated by DataServ, according to a report on ZDnet. It has been operating in the US for a while already.
There are of course machines which are going to be too old to sell in this way, and Apple is offering ways of recycling them without cost. In this way it's co-operating with European directives on Waste Electronic and Electrical Equipment disposal (WEEE) - your local authority's recycling facility is likely to have a palette for disposing of electrical equipment, too.
Apple is pitching this very much as a 'recycling' effort and is of course encouraging people to trade in their old Apple system for a new one. There is no obligation to spend any money you take out of the scheme on Apple kit.
The scheme will also look at old laptop computers of any description, but the values it applies to these are significantly lower than on Apple kit.