I think there are a number of things that could be in play, depending on how things look on Tuesday:
-- The Tories. Even if Harper loses seats, he'll probably get to decide his own timing. Will he want to stick around? That may depend on what the future looks like, based on what happens to the Liberals.
-- The Liberals. Call me crazy, but if Iggy wins his seat in Etobicoke-Lakeshore and doesn't immediately resign, the Liberals may want to stick with him -- even if they have lost seats. There is no heir apparent, and changing leaders doesn't seem to be a recipe for improvement. They may want to give Iggy a chance to grow as a politician and focus on the real problem, which is establishing the party as something different from the Conservatives (in substance, rather than just rhetoric).
-- The NDP and the Liberals. If the two parties have more combined seats than the Tories, they should think about entering an accord and seeking to form government, even if it is headed by Layton.
The best way to hurt the effectiveness of the Tories is to knock them out of power. Parties that aren't in power usually have to find a new leader, have more difficulties raising money, etc. Plus, Harper is threatening to make good on his pledge to scrap the federal subsidies to political parties, so the opposition has good reason to want to bounce the Tories out.