I think Wii will still be in the lead in 2011, PS3 and 360 will basically be a tie.
Wii is easy to eaxplain, it is still selling like crazy and once the best new thing effect wears off then it will be too large of an install base for any one to catch up.
360 and PS3 have both lost there luster among the general public for high tech. 360 will soon be in mass market pricing and PS3 has Bluray movies which will probably get more popular as HDTV become more common in the average household.
2011
Wii - 100mil
PS3 - 50 mil
360 - 45 mil
I would cite regulation, but I know you will simply ignore it.