Assuming that somehow Sony had the do this, and it wasn't a clearance sale because the PS3 was being discontinued.
I would expect a large drop in 360 sales, and a large increase in PS3 sales.
I would expect a slight increase in PS360 sales, even allowing the PS360 to outsell the Wii for that month
I would expect Wii sales = Wii Supply, as always
I would expect a 360 price cut, or extra bundling, or possibly value add such as bigger harddrives
I would expect it would bring the different colour versions of the Wii into 2009 instead of 2010
I would possibly expect it to bring the first, $50 Wii price cut into 2010 instead of 2011