Polls taken beyond an election call are generally not very reliable. I seem to recall Dion neck and neck with Harper (or even in the lead) in the summer leading up to the election.
The reason is twofold.
1) Such polls test general apathy/antipathy and historical leanings, as a concrete decision is not imminent. In this instance, Iggy generally stands for some harping from the sidelines, but no real concrete alternative. In an economic downturn being on the harping side will naturally pull polling in your favour, but it should not be confused with actual intention on voting day.
2) Pollsters can afford to push poll or otherwise not be too careful in their methodology, as a true test of accuracy is so far out, there is plenty of time to become more scrupulous.
Of course, that doesn't stop the media from measuring Harper for his political coffin.