A conversation I had today told me that wrong impressions of what the polls have said this year are rampant.
Essentially, the status quo was upset in early April by the Brault testimony at Gomery. Liberal support began to plunge but was arrested by Paul Martin's speech on April 21. Wild fluctuations in party preference continued as various dramatic political developments unfolded - Martin's deal with the NDP, David Kilgour leaving the Liberals, Belinda Stronach joining - culminating in the squeaker of the confidence vote on May 18. After that we essentially witnessed a return more or less to the status quo ante.
It is all laid out in a graphic here. Note that I have used a three poll rolling average to smooth fluctuations (which I think are meaningless). My source for all these numbers was the table here.
What isn't shown is that after the May confidence vote, Harper's personal numbers plummeted while Layton's soared.