Well as I've said numerous times about Natal, the key to success is going to be the software. The wii remote didn't sell to people because they were generally interested or thought it was a cool idea. It sold to people because when they bought the system they got Wii Sports which was a great game that showed it worked and proved games could be very fun and different using it. Follow that up with other big key releases, some that use it heavily and some that don't, and you have a successful system.
Microsoft will have to do something similar. They need to have a game, probably bundled with it, that shows how good this is and that people should pay the money to buy it. Doesn't need to be a big branded title, like Wii Sports wasn't when it came out, but it does need to be marketed heavily to show it is a big title. I think they can pull it off but Natal is a much harder sell than the wii remote was.
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