6 to 1 isn't considered high unless its so soon into the console's life. In the case of the X360, it's 6+ attach ratio is the highest in history compared to the ammount of time it's been on market.
Eventually, most systems wind up with a 10:1 ratio for consoles. The Xbox, PS2, and GC all reached 10:1.
For the X360 in Japan, I agree that I'm suprised it's not higher. However, alot of people bought X360s for one or two games, then sold them. This really hurts the ratio for it.
redspear - actually, the X360 has led the Wii and PS3 at any and every point for attach ratios in the US by a huge margin. The Wii and PS3 are around 3.5 for sold software units in the US - in August last year, the X360 was around 5.0.