1UP speculates Microsoft has bigger plans for Natal than to just release it as an add-on for the Xbox 360. The site believes Microsoft is actually building a more powerful version of the Xbox 360 that will come standard with Natal, this should be seen as an evolution of the current Xbox console and not as an entirely new platform.
However, the new console won't just be an Xbox 360 with a camera, though -- we've heard it will be considered a new platform and carry a new name (Xbox Natal?). It's not clear what sort of upgrades we might see, but some have suggested it will be an slight upgrade of the current Xbox 360 technology. Current Xbox 360 games would be playable on it, but future games would be able to take advantage of the added muscle. Similar to what we see in the PC space, games played on the more advanced Xbox would look or perform better, but publishers will still be able to support owners of both systems. Some developers have complained that they've already maxed out the Xbox 360; perhaps this will give them room to expand.
Make no mistake, we wouldn't be talking about the sort of hardware leap we've seen with Xbox (or most other) platforms in the past, and we're not talking about Microsoft ending one console cycle and starting another. We're talking about an evolution of the Xbox 360; similar hardware but upgraded, repackaged, and rebranded. It's actually not that unlike what Nintendo did with the Wii, where it essentially took the GameCube hardware, stuck in motion controls, and successfully relaunched it.