I've heard some Eurocrats claiming that mode S will have to be on larger model aircraft, hang-gliders, gliders and parachutists - as well as tens of thousands of microlights and simple homebuilts across Europe.
Oh yes, and if you look at the RF power a transponder puts out, you really don't want to be near the antenna. Fine inside a metal box with the antenna on the outside, but a wooden or GRP aircraft? Best reserved for people who have finished breeding!
G