I haven't yet seen any free tools that scan 802.11a or g networks. I expect some of the currently available stuff to start doing it soon (Netstumbler and Wellenreiter, in particular). Another source to check out are the nifty commercial tools at BV Systems. None of them currently claim to support 802.11a or g, but I'll bet they do soon! Also, they have about the coolest looking hand-held gadgets since the last Ghostbusters movie!
As for devices that do radiation scanning, the BV Systems stuff can do it, but not as a 24x7 solution. The AirDefense solution you point out is designed for 24x7 monitoring. I haven't myself seen AirDefense used in a production environment yet. However, it certainly has useful features in detecting renegade wireless LANs and permitted WLANs without proper security settings.
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This was first published in July 2002