Microsoft has rolled out a number of demos showing how Project Natal can work with existing Xbox 360 games -- Burnout Paradise, Beautiful Katamari -- but the company dashed hopes of Project Natal-enabling patches once the hardware is released. Kudo Tsunoda, otherwise known as the man who demo'd Project Natal at E3, told Destructoid this would be difficult.
According to Tsunoda, it's required substantial coding efforts on Microsoft's part to reengineer these games for Project Natal. A simple patch (as patches have size limits) would not likely be enough for games to suddenly start incorporating Project Natal controls. Already released Xbox 360 games with Project Natal support aren't out of the question, but it's more likely those will come in the form of outright re-releases, rather than downloadable updates.
Maybe there won't be a bunch of patches for existing Xbox 360 games, but is there any reason Microsoft and third-party publishers couldn't release some of the better retrofits as downloable demos included with Project Natal or through Xbox Live? You can have that idea for free, Microsoft.