Seems that this forum has daily posts/queries on which touring bike to buy, with a healthy mix of debates over fine details vs. new riders asking, "I own X. Can I tour on that?"
The new Adventure Cyclist Magazine (April 2009) has a cover story by John Schubert on buying a touring bike. I liked his concise summary, which can summarized in a couple of sentences:
"A lot of manufacturers make touring bikes and they are all reasonably well suited for touring...they aren't all exactly the same, but they're fairly close....
"...the basic requirements: chainstays of 17.5 to 18 inches, a low gear of 20 to 25 gear inches, a frame that fits you, and (the ability to) distribute your heavy camping load between front and rear bags."
There is a lot more to the article, which is worth reading, but I liked his summary.
The magazine also has a nice full-page list of touring bike makers and a full-page discussion of the Salsa Fargo.