The conclusion many have drawn from the combined Halo 3 and Xbox 360 sales in September is software sells hardware, which is certainly true to a point. However, the increase in Xbox 360 sales due to Halo 3 was perhaps not as large as the raw numbers might suggest. Let's take a closer look.
Recall that in August the Xbox 360 sold at a rate of 69,200 systems per week in response to the system's price drop. Let us make the modest assumption that sales would have continued at roughly that same rate in September without the launch of Halo 3, between 65,000 and 75,000 per week. Since September was a 5-week period for NPD, the Xbox 360 would have already sold between 325,000 and 375,000 systems in September without Halo 3. So the effect of Halo 3 was to move between 150,000 and 200,000 extra systems in a month, which sounds rather modest compared to the 3.3 million copies of Halo 3 that were sold.
If the bump in hardware sales carries over to October and beyond, then we can certainly conclude that Halo 3 is definitely moving systems. And no one denies that Halo 3 didn't help move extra systems in September. However, with the one data point from the past month of sales, the true Halo effect has yet to be demonstrated.