Bodhesatva's points, broken down individually:
The 360 has more software market share than hardware market share by a small amount (3%-5%). The 360 has the most hardware sold in North America. These two things combined mean the 360 is the best software mover in North America at the moment. With me so far?
The PS3 and Wii hardware sales have now given them over 50% of the hardware in North America. This means that combined the Playstation 3 and Wii represent a little over half of the hardware sold in the continent. Before this month though the Wii and Playstation 3 represented both less than 50% of the hardware and less than 50% of the software. Now that they represent more than 50% of the hardware they are also representing more than 50% of the software.
What this implies is that while the 360 has a software weekly attach rate advantage over both the Playstation 3 and Wii, it's not as large an advantage as some seem to believe. This implies that when a platform (the Wii for example) passes the 360 in hardware sales by 10% or more in North America, it will also pass it in software sales in North America.
It's also notable that in worldwide sales, the Wii (without a huge hardware advantage) sells far more software each week than the Xbox 360 for the last 4 weeks.
Now that that's been resolved, your complaint is that Super Mario Galaxy artificially inflates these numbers, but every time a game is released software sales are raised, and since the Xbox 360 got 2 releases (CoD4 and Assassin's Creed) which combined had a much larger individual attach rate than SMG, it's clear that a November software comparison is in no way biased towards the Wii's single major release when the 360 had at least 2, and 3 if you count Mass Effect. One of those titles was even out a week longer!