Through March 2009 shipments are something like:
Wii ~50.95m
PS360 ~ 53.65m (22.85m and 30.8m are my estimates for PS3/360)
Through March 2008 shipments were:
Wii ~ 24.45m
PS360 ~ 31.85m
Through March 2007 shipments were:
Wii ~ 5.84m
PS360 ~ 14.51m
So the gap narrowed about 106,000 units per month on average from March 2007 to March 2008, and then from March 2008 to March 2009 narrowed at about 390,000 units per month.
In other words, even as the PS3 and 360 have seen price cuts the gap has generally shrunk between them and the Wii.
By March 2010 it should look something like:
Wii ~ 75m - 86m (24m to 35m units in the year ending March 2010)
PS360 ~ 74m - 82m (20m to 28m units in the year ending March 2010)
I'd probably put it at 80m and 78m by March 2010, so in terms of shipments it will likely happen in ~February 2010. If Wii or PS360 dramatically speed up or slowdown, it could happen much sooner or much later than that. But its hard to imagine PS3/360 shipments of less than 10-14m for the next fiscal year (each), and hard to imagine Wii shipments under 24m or over 35m.