While this isn't something I'll be buying due to my current ownership of an Xbox 360 Elite, I do think that this a great move made by Microsoft in terms of business. While saving $10-$20 per unit might not seem like much, it definitely will save Microsoft some money in the time to come.
I do think, like Caspa said, that they may have some trouble with the price tag. While those who are interested in Natal and are wanting to get in on the action, then the price tag probably seems fair, but to those who are not looking forward to Natal, which is quite a few people as is the same with the Playstation Move, will likely see it as a scam, or simply something that they're not interested in.
More than anything I'm curious to see what it'll look like. Obviously it'll be the basic design of an Xbox 360, but it'll be interesting indeed to see it in its not, well, over-sized form.
Also, if the new slim comes with a new processor and graphics card, that could be a sign that Microsoft is going to try to expand the limits of the Xbox 360, ending the criticism towards the console claiming that Microsoft had "maxed out" the Xbox 360, and that it would not be able to improve further in terms of gaming content.