MS is feeling the heat after dropping behind the PS3 in sales. I don't think this is the answer as there are certainly a number of more significant reasons casual gamers opt for a Wii over a 360, including price and marketing buzz generated by high demand and (artificial?) shortages. New game prices are cheaper too, and if you're a "casual" gamer, price is a huge factor.
As a new casual gamer, I had been hoping to get a Wii, but couldn't be hassled with tracking one down and opted for a PS3 when they dropped to $399 because of the BD playback. Unfortunately, my game options aren't near what it would be if I got a Wii, though I don't regret my purchase.