Actually, I believe the plan is to reduce the lanes to 2 each direction....Burnhamthorpe is where the light rail is supposed to turn off Hurontario - then going north on Duke of York to the bus depot at Sq1...this will probably take many years to be completed though...the light rail will run down the centre of the median....
I am pretty sure there are no plans to widen Burnhamthorpe...
edit: here is the latest master plan for MCC, which was just released on May 7, 2009..../warning 20MB file/
http://www6.mississauga.ca/onlinemap...ng2009May7.pdf