It was have been former Microsoft bigwig
† John Schappert's
dream that Natal "will work with every Xbox 360 sold and to be sold", but don't expect it to be a comfortable fit with older games.
Microsoft Games' Kudo Tsunoda has told
Destructoid that it's unlikely we'll see older games patched to work with Natal. While Microsoft has been able to demo Natal with older games such as
Burnout Paradise, that apparently required a lot of tampering with the code. In order for publishers to make older games compatible with Natal, they would likely have to re-code then re-release them.
†Vice President of LIVE, Software and Service, to be precise.