Harper has been able to cling on to power for several reasons: After three elections in just over 4 years the coffers of the opposition parties are exhausted (as in the UK, the Conservatives' backers have deeper pockets), and so is the public's enthusiasm for election campaigns. Another factor is that the Liberals, the main opposition party, are going through a prolonged identity crisis, and have little enthusiasm for power or clear ideas about what they'd do with it if they got it.
The result of the last election in October 2008 was as follows: Conservatives 124 seats, Liberals 103, Bloc Québécois 51 seats, and NDP 29, so the Bloc Québécois hold the balance of power. In the weeks after the election the Liberals and NDP discussed ousting Harper from power by forming a coalition, with the Bloc agreeing to provide support on confidence issues. What happened next was an extraordinary prorogation of Parliament by Stephen Harper, and by the time that Parliament met again in January the Liberals had a new leader (Michael Ignatieff) and had withdrawn from the proposed coalition.
So what comparisons can be made with the British situation? Firstly, there is the constitutional point that unlike Harper in '08, Cameron is not the incumbent Prime Minister and so if the Labour and Lib-Dems do form a coalition he cannot insist on forming a minority government instead. The Queen cannot ask Brown to step aside for Cameron as long as Brown has the confidence of the House of Commons; but what is the constitutional position if the Queen is asked to accept Gordon Brown's resignation and choose between Nick Clegg (or someone else) as coalition PM, or Cameron as minority government PM?
In Canada, opinion polls suggested that there was a lack of support for a coalition, even among Liberal and NDP voters; the media had been talking up the dangers of coalition government in much the same way that they are doing here. Any proposed Labour/Lib-Dem coalition is likely to encounter a similar lack of public confidence compounded by Gordon Brown's personal unpopularity so it may not be enough that L + LD = > C.
Presuming Cameron succeeds in forming a minority government, his next hurdle is to get a budget passed. In Canada the Liberals have preferred to support the budget to forcing an election. However that is easier to do in Canada than here for two reasons: Firstly Canada is a federal system and the provinces have their own tax raising powers, so the federal budget accounts for less than half of public finance. Secondly Canada has a highly regulated banking system, and only been minimally affected by the worldwide recession, so Ignatieff has not faced the choice that Nick Clegg might face of not opposing a Conservative budget which is going to provoke riots in the streets, or forcing an election which might be equally damaging politically and economically.
Stephen Harper is often described as "Bushier than Bush", but minority government has made it impossible for him to fulfil his right-wing agenda in domestic policy. In foreign affairs however, he has more freedom and, to the embarrassment of most Canadians, Canada has played a role in climate change negotiations equivalent to football hooligans at an away match, and is now blocking the international bank tax proposed by the IMF, and excluding abortion funding from international aid. Cameron is not nearly as scary as Harper, but his bizarre choice of allies in the EU does not bode well.