Considering a very small handful of Wii games (like Mario Kart, Super Smash Bros., Wii Fit, Wii Sports Resort, and New Super Mario Bros.) sold more than 125 million copies alone, I assume the total software sales - and thus, the attach rate - *doesn't* include downloadable games.
However, I don't like the attitude that downloadable games shouldn't count as "real" games.
And there was a study from a couple of years ago that showed Wii owners played games only 30 minutes a day less than Xbox 360 and PS3 owners. That doesn't seem like they're only playing games once and then putting them in storage, but who knows?