Actually, looking at the software ssles compared to the hardware sales of the 360, Reggie was right. Only around 8% (Rough math here!) or so of Halo 3's sales were from new hardware owners, the remaining 92% of sales came from people who already owned the console!
Considering the fact that there is no shortage of 360 consoles to go around, the 360's H/W sales should be higher really (Though I reckon quite a few more new owners did jump in to get Halo 3 by picking up 2nd hand 360s, but we'll never know whether this is true or not...)