[...] in the early 1900s, radio equipment was easily thrown off when signals of the same frequency from more than one source overlapped. We call this phenomenon "interference." In fact, the waves sent out by different transmitters don't interfere with each other at all. They pass right through each other unchanged.
Interference occurs in the receiver, when its antenna picks up multiple signals of the same frequency and has trouble telling them apart. In other words, interference is a function of the intelligence designed into the receiver, not a function of what happens in the airwaves -- and receivers can be a lot more intelligent than they were 90 years ago.
He goes on to show how unregulated spectrum could lead to all sorts of innovations. And it already has -- Wi-Fi (a.k.a. AirPort and 802.11b) wireless Ethernet networking, those two-way family radios, and cordless phones are just three examples.