Here is the problem as I see it ... Rock Band was a highly hyped, high quality game that had massive cross-over appeal between (so-called) "Casual" gamers and "Core" gamers and was amazingly successful; and yet it sold an order of magnitude less units than people are expecting from Natal.
There is no doubt in my mind that given enough time (a couple of months) Microsoft will be able to sell 1 or 2 Million Natal-Systems to people who own XBox 360s if it is a reasonable price and they have a couple of games people are interested in playing. The problem comes in getting people to buy an XBox 360 and Natal, especially if they already own an existing console. Basically, getting people to spend $400+ at launch is pretty easy because early-adopter types will spend a ton of money to have the latest and greatest technology; but 4 years down the road when all early adopters have already bought in, expecting anyone to get (super) excited about spending $400 seems unrealistic.