Paul Wells offers his thoughts on what might happen if the Cons lead in the seat count in a minority Parliament. But I'd think it's worth noting two other considerations to counter Wells' take that the Cons could hold on with substantially less than half the seats in the House of Commons.
First, particularly if the combination of NDP and Lib seats adds up to a majority, it's hard to see which of the parties running flat-out on the need for change could possibly do anything but vote down the Cons' first throne speech.
The issue isn't one of relying on technical arguments against fairness and public opinion, but acting out of fairness to supporters based on their clearly-expressed opinion. Any action which kept the Cons in power would be guaranteed to alienate the large mass of people who want to vote Harper out more strongly than ever, while allowing the other party to be seen as the more effective opposition as another election loomed.
Which leads to the second consideration. As I've noted before, if Stephen Harper stays in power, then Stephen Harper retains the ability to call another election any time he wants it. And that's not in the interest of either of the parties who are already being squeezed by a prolonged election campaign.
Of course, Harper would have the constitutional entitlement to try to cling to power. But I'd think the chances of the Cons holding onto power past a throne speech in a minority Parliament would be limited to extremely narrow sets of circumstances - perhaps including the Cons being able to induce MPs to cross the floor or at least leave their existing parties, or winning the support of a Bloc presence sufficient to get them over a majority threshold. And if the NDP and the Libs between them have the ability to bring about a change in government, both principle and politics dictate that should be the result.