Personally, I don't know of modern systems that do revisions that speed up the system and make it faster, etc. etc. When a company like sony comes out with their slim versions, the only specs that change are size and power consumption. I do remember an upgrade that came with Donkey Kong 64 for the N64...but personally can't remember any since then.
I highly doubt if xbox would release a slim version, it would have newer, better equipment in it. If it were to have newer and better equipment in it, one could argue it isn't even the same system. It would in theory be the Xbox 360 v. 2.0. Not to mention, knowing microsoft, whether or not this would be a cheaper alternative, they would not release a new version of the xbox that is better and faster, and then charge consumers the same price for it. They charge $100 for a wifi piece for your 360, you honestly think they will charge the same or lower price for upgraded equipment...
Also, I do believe that if Natal comes out in the way that it should, and works properly for games, etc. etc. Then, yes, Natal is to the 360 what Wii is to the Gamecube. Unfortunately, I don't think it is physically possible to have simple movement controls without a controller for the 20+ controls that you see in a shooting game. How about a game like Street Fighter...you really think you are going to do extreme moves that often (in playstation) will involve pushing r1, r1, triangle, circle, a complete spin with the analog stick, three quick taps of triangle...I have no clue if that is an actual move, but the hard ones do look a lot like that. It just doesn't seem probable that it will be able to handle games that require such precision like this. If you want to take this precision away, it also takes the fun of the game away...the best moves are supposed to be hard, but how hard can they possibly make it before Natal will not recognize what the heck is going on?
Money can't buy happiness. Just video games, which make me happy.