So thats how it works
Just by accident I was dangling my iBuds over my PowerBook and I noticed that they were attracted to a spot about half way down the screen on the right hand side. Being an inquistative sort, it tried on the other otherside, and then tried on the corrensponding point on teh keyboard. The thing went to sleep!
I'd always assumed that the Hall field detector was near the latch, but its actually under the keyboard! Cool!