I want to reiterate the point made by Hugh Jampton back at 10:01 AM -- the guitar may be a comparative newcomer, but what about older and very similar instruments like the lute, the cittern, etc., not to mention the mandolin, balalaika, bouzouki, etc.?

One might object, I suppose, that old-time lutenists may have played arpeggios and countermelodies rather than rhythmic chord progressions in the style of beginning guitarists. However, as many above have noted, a sophisticated musician can use the guitar in any number of different ways, not only as a chord-comping machine.

Backing off to the more basic question of "What is folk music?":

One school of thought is that a "folk" player's mission is to duplicate, as closely as possible, the sound and flavor of some long-gone historical time and place. An admirable goal, to be sure, and the main impetus for collecting and preserving many wonderful songs and tunes.

One problem inherent to this approach is that we have no idea what any musical performance actually sounded like prior to the introduction of recording technology. Traditionalists who are aware of this limitation can do an excellent job when they try to imagine and to recreate a plausible facsimile of how a piece might have been performed and how it might have sounded sometime back in the distant past. However, many more naive individuals feel it is necessary to slavishly imitate the oldest ("most authentic") recorded version of a given number, which necessarily dates back only to the first quarter of the twentieth century.

The opposite point of view is that "folk" music is whatever music is familiar to one's own culture, to the folks to whom and/or with whom one is playing. This theory is based upon the observation that music (of any kind) exists ONLY IN THE PRESENT MOMENT. Since we all live in the modern, industrial, internet-connected world, our shared musical world is ridiculously eclectic and inclusive.

From this point of view, our current real-world "folk" heritage includes Beatles songs and garage-rock stuff like "Louie Louie" as well as Child ballads and 1930s Delta blues -- and the truest folk musicians are the buskers, today's "songsters," who have to give the people what they want (usually a mix of the player's own favorite pieces, many of which may be unfamilar to the audience, along with crowdpleasing pop-culture faves).

Then there's a sort of middle ground, where traditionalists play time-tested old music from a particular well-defined national (or other) heritage, using contemporary instrumentation and *some* degree of modern accompaniment style, but making a strong effort to retain the overall flavor (e.g., a "modal" sound) of what we know of the old or "original" style.

Finally, returning to the original question, what *about* the guitar?Someone above mentioned (was it you, Mooh?) that the guitar is "the people's instrument," easily playable at a very basic level but also capable of great subtlety in the hands of a true artist.

So, while guitar accompaniment is certainly not *always* desirable or appropriate, it certainly cannot be ruled out as automatically "inappropriate" for whatever might be defined as "folk music."