I assume that by direct competition the meaning would be that all customers are choosing between either a wii or a ps360. But I think that would be a really strange thing to measure.
True enough, the PS3 and the 360 are basically competing for the same customers. They are essentially the same thing, really. Most of the games are the same, and both sides have a couple of exlusives.
What nintendo has done however, is to expand their market. A lot of people that would not be interested in the MS or Sony consoles are in fact very interested in the Wii. This bigger market makes things a bit more difficult to measure.
I'm guessing what you would want is to figure out which Wii consoles bought was in that consumers mind competing with a PS360. That would be impossible to figure out. So the question is, in the 'traditional' gaming market, what is Nintendos market share? Answer: probably impossible to tell, especially by looking at sold consoles. If you look at what type of games are selling and how, I believe you with a little reasoning and tons of guessing could get a somewhat clearer picture of how nintendo is doing with that part of the market (though I haven't tried and won't try any guessing).
This sort of divisioning of the market would probably make the most sense. 'Casual gamers' and 'hardcore gamers', if you will.
But even though it would be the best way to divide it, I do not really think that there is reason to do so. How one calculates marketshare is different from person to person, there are always as many ways to divide a market as there are people. Consoles are probably competing against the PC in a way, and gaming in general is still competing with tv, sports, movies and other pasttimes for the money and time of people. So what IS the market is the first question. Once you answer that, you can start counting your competitors.
This is why the whole marketshare thinking is ridicoulus (argh... spelling). The best way to count success is how much money you are making. The second best is to count how many consoles you have sold (which is what VGChartz are incidentally doing).
Nintendo showed this when they launched the Wii.
They are selling to people that have never been counted as part of the video-gaming market at all. This means that basically, all the machines previous to the wii could have actually been sold to these people as well, if they had had the right features. These machines (as well as the PS360) did therefore, in my mind, miss out on a considerable part of the 'actual' video-game market.
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