+1 to
sourdoughhome.com. Read eveything in it, and try to read most of his references.
I've messed w/ starters for years, but I've never tried the grape method. If you're going to keep a starter, you've got to be serious about it. It's like having a cross between a pet and house plant.
My theory is that you can start you're starter a number of different ways to get it established, but eventually, if present, the naturally occuring yeast in your area will take over and you're starter will develop it's own unique taste based on that. If you don't have much/any naturally occuring yeast (I live in a high alpine desert environment, that's hot and dry in the summer and cold and dry in the winter, and I don't think I've got much to work with here), then you're cultivating and trying to maintain what ever you started with.
Spike - I'm not sure that the grape juice method will work. As said above, the yeast on grapes is on the skins, so using the just the juice you may be making alcohol. In contrast to what Wenus said, when I started mine I fed mine twice a day, discarding half each time, for about two weeks (as per sourdoughhome.com's instructions). Once established, I keep it in the fridge and use at least half at least once per week, and fed w/ equal parts water & flour plus salt and some type of sugar (cane sugar, honey).
But alas, after about 9 months or so of this method, mine recently turned red, and I think I have to chuck it. It the past several weeks I've been adding/using different types of flours/grains (brown rice flour, buckwheat flour) for feeding, trying to avoid gluten, and as an experiment. I think something about the type of flour/grain I've been feeding it may have allowed other bugs to over populate.