Eric at 308 takes a shot at adjusting polls for voter turnout trends.
First, I looked at the last week of polling in the three last elections. In 2008, the average result of that last week was 34.2% for the Conservatives, 26.6% for the Liberals, 19.5% for the NDP, 9.6% for the Bloc Québécois, and 9.6% for the Greens. The actual election results were 37.7%, 26.3%, 18.2%, 10.0%, and 6.8%, respectively.
In other words, on election day the Tories had 110% of their average polling result, compared to 104% for the Bloc, 99% for the Liberals, 93% for the NDP, and 71% for the Greens.