I'll leave in-depth analysis to someone more suited to the task, but I did a quick analysis of American software for 2007 on the Wii and this is what I found:
In the year 25,973,103 pieces of software were sold (not including Wii Sports and Wii Play)
10,029,985 (39%) of this software was software published by Nintendo
15,943,118 (61%) of this software was published by third parties
This means that (roughly) 3 titles were purchased by every Wii owner in 2007
There were 50,160,001 User Owned Wii Months in the Year
Wii owners (on average) buy 1 Wii game every 2 months