Wouldn't it be utterly awsome if Microsoft/Rare did something crazy like make Perfect Dark for the 360 and you can use the DS for additional Features? Kinda like the GBA->Gamecube?
No, actually it wouldn't. While I will not argue that handheld to console connectivity opens up unique possibilities for game design, the masses just have not yearned for it and there have not been too many games which have utilized it. The big thing for Nintendo a while back was the fact that the GBA could be linked as a controller for the Gamecube. A few games used it, but it was too expensive for consumers unless they already had the equipment (and forget four player Crystal Chronicles...).
With the DS being able to play GBA games, it would have been easy for Nintendo to either include a link port on it, or offer a Wavebird-like wireless adapter so that the DS could still be used in conjunction with the Gamecube. However, Nintendo effectively decided to drop the handheld to console connectivity. Even Sony tried the same thing with the Pocket Station, but felt that it wasn't even useful enough to debut in the US.
The main problem with this kind of connectivity is two-fold. First, a lot of people are not willing or able to spend the kind of money required for full connectivity and feel cheated when they buy a game that they cannot fully use. Second, connecting the handheld to the console requires (typically) that you then use the handheld as the controller. In essence, you need to have two different control schemes for a game (regular controller and handheld device) since the button configuration is completely different. For example, by using a DS on any of the current consoles, you lose the analog sticks and a lot of analog button functionality.