I figured the 3d frequency overlap bit out in 2000 while consulting for the 802.11 design for dorms for a local university, having recalled the way that having the transmitter on a different floor mostly blocked the signal with early freq. hopping 802.11 a couple of years before that. :-)

It's great that this works, but it just points up the falsehood of some of the claims from the free spectrum camp. It's fast becoming conventional wisdom that bandwidth somehow expands as more transmitters are added to the system. WiFi supposedly demonstrates that smart radios are no longer constrained by the old physical limits of spectrum availability.

The reality is that WiFi works well exactly when there is little contention for its part of the spectrum. Where you do have congestion, as in the cases described by the linked article, you have a problem, just like with every other radio based technology. Being able to cram the transmitters onto frequencies a little closer together will help ease the crowding in those areas where it's a problem. Moving to the less crowded 5 GHz bands will help too. But ultimately WiFi, as with other radio technologies, is limited by spectrum availability.