So, according to Wikipedia's entry on the Parliament of Canada, Harper will get to form a Conservative government essentially because it is tradition that "the Governor General [appointed by the reigning British monarch] requests the person most likely to command the support of a majority of the House of Commons (usually the leader of the party with the greatest number of seats in that House) to form a government."
If, in theory, the Liberals, NDPs, and Greens Bloc Québécois got together today, and worked out a quick bargain to form a coalition against the Tories, could they petition to have their chosen leader (presumably whoever is replacing Martin as head of the Liberals) appointed PM? Or is the procedure of handing the PM spot out based on plurality-of-seats strong enough that they'd have to go through the formality of a no-confidence vote and another election (which would risk eroding their seat count, if some people saw them as being "sore losers" and incurring extra expenses)? What would happen if, to avoid that, they just stonewalled Harper's administration on all issues, in the House of Commons?
I really wish they'd offered the international version of Gov't and Politics AP, at my high school, instead of only the US version. A bunch of the students actually petitioned to have that replace the US-only one, because we felt that a) the US one would reiterate too much material that we'd covered in Civics G/T, and b) the international one would look better on college applications -- it would show us to have a more informed, less parochial education. The principal turned us down. As it happens, she was a rather narrow-minded, parochial sort of person.
(ETA: I originally was thinking that the light blue in this diagram was the Greens -- in retrospect, this seems silly. It's actually the separatists from Quebec. I imagine they'd be far less likely partners for the Lib/NDP alliance than the Greens might be. Anyways, again, this just goes to show how we Yanks fail to get even the most basic primer on our neighbors' governments. This really should be remedied.)