Here's a nice chart to look at showing weekly sales: http://vgchartz.com/hwcomps.php?cons1=Wii®1=All&cons2=PS3®2=All&cons3=X360®3=All&start=39040&end=39530
There have been only two occasions when the Wii fell behind in worldwide sales, and neither lasted more than two weeks. The first was the PS3 European launch (March 2007) and the other was Halo's release (Sep. 2007). I think what's also interesting is how the PS3 and 360 have been tracking at a nearly identical level throughout their lifespans so far, despite a great deal of rhetoric on both sides.
End of 2008 totals: Wii 42m, 360 24m, PS3 18.5m (made Jan. 4, 2008)







