I personally think that Skyward Sword will sell less than the Wii version of Twilight Princess.
The Wii's competition gets fiercer and fiercer every year. In 2010, you have to compete with all the new 2010 titles, but you also have to compete with evergreen 2008 and 2009 titles. Essentially, the amount of games that people could potentially buy increases.
And, with the Wii, the amount of software isn't increasing. It's stuck at selling around the same as in 2008 and in 2009, and it's quite clearly going to sell roughly the same in 2011 and 2012.
More titles that buyers could potentially buy, coupled with no increase in software sold, means that the average title will sell less. Essentially, all titles will sell a bit less on Wii now than the earlier released ones.
We see this pattern being true for all installments of Zelda on machines. The second one on the machine always sells less.
So, I'd say somewhere in the 4-5 million range seems likely.
