Couple of things:
1) Wii is a totally different market. IMHO the X360 is largely hardcore gamers who like FPSs - call the generalisin, but there's evidence to back this up....etc
A lot of this will be down to how well it's marketed and packaged, but I doubt most of the current X360 gamers really care at all about this...
I don't altogether disagree... it's all down to the marketing and whether or not the games at launch are able to appeal to both the current 360 owners and potential 360 owners.
However, to catagorise X360 owners as FPS freaks is maybe a generalisation too far. Hardcore, probably yes, but I'd say Shooters, Racers, Guitar Band and Sports are the big sellers. And if Natal can tap into any of those markets, it has a great chance of success. On top of that, puzzle games over XBLA have generally been very popular.
Indeed, even if it's just a case of the 360's GUI being able to be controlled "Minority Support" style, that will have an appeal to a lot of those hardcore gamers as they'll think it's "cool".
What I'm saying is the casual market is already owned. What reason would the casual gamer have to buy X360 + Natal over the Wii/PS3?
As things stand (casual gamer market):
Wii
PS3
X360
If MS want to widen it's appeal (where they keep trying but largely failing - see their camera and Lips/scene-it), most casual gamers will think it's an eye toy or will already have a Wii...so no need to buy...their main concern (initially) is surely the 35M (or whatever it is) customers allready on board.
With regards to the Casual market, clearly the Wii holds a huge lead. However, I'm not convinced that the PS3 has attracted much of a casual market at all this generation. The vast majority of the top sellers on the PS3 are of the hardcore "shooter, fighter" variety, along with the same smattering of Guitar Band and racing-type games. Titles such as Singstar and Eyepet haven't really gain any significant amount of traction, just as Lips, etc haven't.
I''d contend that it would be just as easy to say the 360 is more "casual friendly" than the PS3 as it would be the other way around.
However, it's pretty much a meaningless comment to make, as both usersbases are primarily hardcore.
And this is something that Sony recognises. The Arc isn't designed to appeal to the casual market. All of the promises of 1:1 pixel mapping and unprecedented accuracy with user control show is something that is of no interest to the casual market, which is why Sony is appealing to the current hardcore fanbase with the product. And, with the right product (and marketing), it will sell well, but it's not designed to expand the PS3's market outside of the current usersbase.
As you pointed out, NATAL is something of an anathema to the majority of the current 360 userbase and MS are taking much more of a gamble in attempting to appeal to a wider audience. However, they are probably working on the assumption that there are quite a lot of "Dual console" families out there, who have both a Wii and a 360 (just as there are with Wii/PS3 combos) that there is a ready-made market there in which they can gain some traction.
But outside of that? They need a killer-app that'll appeal to the casuals, and I'm not convinced that MS have it in them.