I think the generation is still up for grabs in all positions.
With the 360 and PS3 getting motion controls that will put massive pressure on Nintendo. Arguably Nintendo is in the most difficult position. They don't have HD gaming and the third party AAA support isn't great - No Final Fantasy, no GTA, no BioShock, no Tekken, no Fallout, no Dragon Age, etc.
Nintendo still has price to work with, but, so does Microsoft and Sony. By the end of this year the landscape of sales may completely change for all three companies... I see scenarios where any of the three can drop and I can see scenarios where any of the three can rise.
To me Nintendo looks to have the most difficult road ahead simply because the graphical and AI capabilities of the system cannot match either 360 or PS3. And third parties haven't shown a lot of AAA support to Nintendo.
My prediction is: the places for WW weekly sales at the end of this holiday season will be the final places at the end of the generation. Today, without knowing the holiday lineup, it is impossible to predict. Microsoft still hasn't revealed anything for the holiday other then Halo and Fable, Sony probably hasn't revealed all their games, and Nintendo hasn't said a word.
E3 will be the first opportunitiy for accurate holiday predictions.