I'd like to begin by saying how much I enjoy Adventure Cyclist magazine; the features are always interesting and well-written. It's particularly good to features by venerable cycling writers like John Schubert, who I always enjoyed reading during another bike magazine's hay day in the 1970s and early 80s.
I do believe, however, that Mr. Schubert may have missed a category of touring bike in his otherwise excellent article "What Kind of Bike Should You Buy?" in the February 2007 issue.
While there are situations that call for a fully-loaded mountain bike on tour, it seems to me that a serious off-road long-distance cycle tourist would be best suited with an "expedition bike". With a steel frame, 26-inch wheels, wide rims, and either dropped handlebars or straights, these bikes are designed for navigating rough terrain and carrying heavy loads with aplomb - there's really no need to use a mountain bike that's been expressly designed for another purpose. The expedition bike makers with which I'm familiar are all English: Thorn (a large selection from St. John Street Cycles; Roberts (the Rough Stuff); and Orbit (the Expedition).
I ride a Thorn Raven Adventure Tour, which is equally well-suited to road and unpaved surfaces (I especially appreciate it in the winter months as road conditions deteriorate). This is one of Thorn's many models designed around a Rohloff hub, another wonderful product that you might consider mentioning in future buying guides. They're growing in popularity, thanks in large part to Thorn's efforts, but mainly because more and more people are falling in love with their versatility, simplicity, ease of use, and low maintenance.
I believe that your readers would find some mention of the expedition bike category of interest, and I also think they could benefit from coverage of the Rohloff phenomenon.