I'm going to start off with the disclaimer that this is purely anecdotal and that this post is not guaranteed to be representative. Most observations come from university housing in Texas.
I see the Wii being almost as strong as the 360 in the 18-25 demographic (the 360 being something like 6-10 in a dorm of 200), with absolutely no female ownership of the 360 and 80/20 ownership of the Wii. Large numbers of people express interest in the Wii, wanting to buy one as soon as things settle down or as soon as school starts, etc.
Several people have reported that their parents have bought Wiis.
To my knowledge, I've yet to meet an 18+ who owns a PS3. I've heard several stories like "oh my uncle and aunt bought my spoiled little cousin a PS3 for his birthday" and similar. I've never even heard a positive opinion about the thing, with the price being the source of the vast majority of negative opinions. Girls in particular don't seem to think that more than $300 is at all worth it for a toy.
To continue on more theoretical ground, and at risk of being as misguided as certain others have shown themselves to be, I'd suggest that the primary market for the PS3 in the US is the 8-18 crowd. The general feeling I've picked up after three years around large groups of 18-25s who live together is that nothing beats Halo as a game to stay up late playing against each other and that nothing beats Smash Brothers, Wii Sports, and whatever topical games are released for parties (some friends of mine borrowed my Wii, rented the Harry Potter game, and went to a fairly large Harry Potter party that largely centered on the Wii, they say). Zelda is popular too.
The male college crowd has a tremendous appreciation for Live and Halo, and the value of certain Wii games as party games drives adoption among an age group that tends to live together in groups of several hundred.
PS2s abound, but I don't know a single person with one who would even consider buying the PS3. Most seem to be eyeballing the Wii.
I would blame the Wii's popularity in this age group on the increasing frequency of long-term relationships and the revelation among male gamers that girls like to play too, just not Gears of War, as well as the ease with which 4+ people can be gathered, along with drinks, for an hour or two of swinging arms around like idiots.
It doesn't hurt that the Wii almost always makes a good impression on guests who play it while a college dorm is just about the most densely packed collection of likely players you could get.
