Interesting that someone revived this thread -- it's the first one I posted to, back when I had a name rather than a handle. Ah, but I was so much older then...
The problem with definitions is that they rely on other words, which we then have to define. I often hear "folk" defined with reference to "traditions" (as Frank Hamilton has done); I think it is interesting that we will argue endlessly about the meaning of the word "folk," but seem to assume that we share a common definition of the word "tradition".
Some seem to feel that a tradition needs to be hundreds of years old, but I think this assumption ignores the fact that life runs at a much faster clip now than it used to, and everything -- communications, technological developments, population growth and migration -- occurs much more quickly than it did in the old days. Since all of these things are factors in the development of traditions, I would argue that traditions can also arise and become established much more quickly than they used to.
In my opinion, Woody Guthrie was primarily responsible for the establishment of a white American singer-songwriter tradition that took hold in the space of just a few years in the 1930's. It drew on a number of other traditions -- some much older (the British ballad tradition, by way of Appalachia), some not so much older (cowboy songs), some comparatively recent (black blues). But there was enough about it that was unique that, in my opinion, it established a recognizable genre of its own. This genre resonated with enough people that a form of folk music tradition was born right then and there, and has continued to flourish to the present day. I think something similar happened with early rock and roll, which also drew together the threads of other traditions to form a recognizable genre of its own, practiced primarily by a relatively distinct sub-population (young postwar males with southern roots from the lower rungs of the economic ladder). I would consider both the Guthrie singer-songwriter tradition and the early rock and roll/rockabilly tradition to constitute "folk" music. [However, the latter so quickly and dramatically achieved commercial success that it was soon wrested away from its folk roots.]
I don't necessarily expect a lot of people to agree with me on this, and I recognize that there are still plenty of blurred boundary lines and opportunities for hair-splitting. But, despite all the lingering disagreements and general fatigue that this topic induces, I still think it is one of the more interesting subjects we discuss on this forum.