I think Madden has reemphasized a valid concern about the Wii console. The concern that many have brought up. Perhaps the Wii audience is just not interested in mature games. Looking at the sales of other mature or conventional games on the system you would not be surprised by the sales of Madden on the Wii.
The sales of Madden actually make you question the legs of the console. Yes the Wii is geared towards expanding the market however. The console has not attracted main stream gamers or at the very least not enough to create market impact. The slow adoption of the console might be covered up by the unconventional or the new gamer joining the greater community. The question is how deep is that barrel. Can Nintendo dredge up twenty million more non gamers? I think at some point sooner rather then later the new breed market will have to start drying up.
Regardless of the outcome the poor sales of Madden on the Wii actually hurts the console in the eyes of developers. Just one more piece of evidence that the console is only for casual, young, or new players. That means the console will get fewer mature complex games. This could lead to a polar offering on the console.