Aaron Greenberg responded in a calmer manner, saying "I think part of the excitement about this announcement is that in many ways we are delivering a next generation experience this generation. With that said, I can confirm that Natal will run on Xbox 360 so no new console investment will be necessary."
Conspiracy theorists should note that both of these denials leave the door wide open for the rumor to actually be true. No new console investment will be "necessary" for Natal -- but it certainly could be an option, and there's always the possibility of Natal technology being built into the next console. And "the right thing for Natal" could certainly be "building a whole console around it." Releasing it as a standalone doesn't preclude the release of a later console with it built in.
Meanwhile, TeamXbox claims to have inside information about plans for the next Xbox console, specifically that Microsoft will not launch it in 2010 because the company is planning to include stereoscopic 3D functionality in full HD, and is waiting for the proliferation of televisions that support that technology.