BDL has rough-sketched some of the info I learned about santoku while in Japan. Let me amplify.
First of all, the word "traditional" should be avoided here. Of the various knife styles still in functional use in Japan, they basically come in three rough groups: seriously old, pre-Western, Western-influenced. (Butchering knives are a hideously complicated and ideologically fraught subject, so I leave them aside.) Since the earliest Western-influenced cutlery is somewhere a little before 1900, and new developments continue periodically, it's not really reasonable to classify Japanese knives based on whether they are or are not traditional.
The seriously old knives still used extensively are the nakiri and the deba. The nakiri is very similar to a Chinese vegetable cleaver: a rectangular blade, not terribly tall (by contrast to the Chinese cleaver we usually see), and usually with a slightly curved edge. The edge is usually ground on both sides, but they are sometimes found chisel-ground --- literally so, meaning that someone just grinds on one side until the edge is sharp, then deburrs, but there is no attempt to hollow out the back face as on a professional single-beveled knife. The deba is a fish-breaking knife, with a pronounced curvature, a somewhat curved and very thick spine, and a great deal of weight. The edge is normally single-beveled in modern times, but there is excellent reason to think that knives like this originated chisel-ground.
The pre-Western knives are essentially a development that arose with a rise in urban "pleasure culture" and the concomitant ability of cooks to command the sorts of money that would commission very good knives for their purposes. This led to better steels, better shapes, and the hollow-grinding we know in single-beveled knives today. The important knives that arose in this context are the usuba, deba (single-beveled), yanagiba, and takobiki. There are many regional and technical variations among these, but all have in common the true single-beveling method with a hollow-ground back. The usuba has a dead-straight edge and is used for vegetables, replacing the nakiri in a professional context. The deba is simply a technical refinement on its predecessor, and chisel-ground ones continued to be used widely. The yanagiba and takobiki are somewhat differently-constructed sashimi knives, associated with the Kansai (Kyoto) and Kanto (Tokyo) regions, respectively.
Once Western influence comes in, you get the Japanese knife makers experimenting with truly double-beveled knives, which had not been used to any significant degree, in culinary knives at least, for quite some time. The first step is simply to copy Western knives for the use of Western and Western-trained cooks. This is where you get the gyuto, for example --- it's a chef's knife. But then something odd happens.
Now speculation must come in strongly, because documentation here is scant and ill-researched. What I think happens is that knife-makers and non-professional cooks start noticing this odd (to them) thing, that Western knives are not strictly segregated. A Western vegetable knife isn't a "vegetable knife" in the sense that it can never ever touch any kind of flesh, but rather a knife that is particularly well-suited to cutting vegetables. By contrast, a nakiri or usuba never touches flesh, ever. (It's unclear whether this segregation also covered the use of the heel of a deba for heavy herb mincing.) The other thing that happens is that suddenly everyone can feel free to eat meat openly, and in fact all of a sudden it's stylish and clever to do so. The first chicken restaurant in Kyoto opens in 1880, I think, and by 1900 things like yakitori (grilled chicken on a stick) and teriyaki and sukiyaki and so on are all the rage. (If you've ever had tonkatsu --- crumbed and deep-fried pork cutlet --- and wondered why the sauce tastes so much like Worcestershire, that's because it is: katsu is an attempt to say "cutlet," thus "pork cutlet," and the sauce is supposed to be the Western universal sauce sort of like the East Asian soy, and they thought that sauce was Worcestershire, because who were they talking to?)
So now, roughly 1900, the housewife needs a new knife, one for cutting (eek!) meat. The thing is, her mother never had such a thing. And you know what? Knives aren't cheap. So somewhere along the line somebody says, "hey, ladies! You don't need all that junk, the nakiri and the deba and all that old-fashioned nonsense. You're young and hip and chic and Western! You need a Western knife, one that can cut anything! And here you have it!"
What gets sold this way is sort of like an unholy marriage of a chef's knife (gyuto) and a nakiri. Presumably it was thought that this shape would be reasonably comfortable to a lady who'd grown up with a nakiri, but it was truly double-beveled and curved and pointed like a Western knife and could thus be used effectively for meat as well as veggies. Fish, well, the deba still reigns, but two knives isn't half bad, and after all you can buy cut fish from the fishmonger.
This knife eventually settled down with two principal names: santoku and bunkabocho. "Santoku" does indeed refer to the three Buddhist virtues (of which there is more than one set), and seems to have been a marketing device: "this knife is thrifty and Japanese as well as clever and Western!" Bunkabocho literally means "cultural knife," but actually the term "bunka" had in the 1910s-20s a connotation of "clever, modern, Western," and thus this is the "clever, modern, Western knife!" Competing marketing terms, really.
In many respects I think Rachael Ray is pushing the knife precisely as its inventors would have hoped, as a thing for up-to-date would-be-chic middle-class housewives who (want to imagine themselves to) have better things to do than futz around with knives. She was hardly the first: my sense is that these knives first made significant inroads through Global knives in professional kitchens, and eventually word leaked out. People in that context were captivated by the knives' durability, sharpness, and lightness --- qualities that actually had 99% to do with their being decent-quality Japanese knives rather than santoku as such. Now the upper-middle-class is hot to trot on santoku, and there are even people who think the nakiri is really the cool kids' thing, where the professional kitchens have been discovering good-quality gyuto and single-beveled knives --- and thus get pleasure out of sneering at whatever their fashion-conscious customers buy.
I hope this historical context is of some value to all.