According to Airline Route Update (Jimyvr at Airline Route is a great source for this info!) United will be adding a third daily flight between Toronto and their Denver hub as of Feb 11, 2010. The flight will be operated by United Express with an Embraer E170.
New flight timings:
YYZ-DEN Dep 1500 Arr 1650
DEN-YYZ Dep 0840 Arr 1350