Ain't gonna happen.
Not a chance.
As much as the NDP would love a PR referendum, they couldn't even get that deal out of Paul Martin when he was desperately trying to cling to power in 2005. So given the Liberals are maybe-kinda thinking about voting down Harper sometime in the fall, it's not like he has a gun to his head.
And it makes absolutely no sense for Harper to sign on. Since the rise of the CCF, first past the post has been exceedingly kind to the Conservatives. They won the only two Gore Elections (lose popular vote, but win), in 1957 and 1979. And with governing coalitions the norm in a PR system, it has to concern Conservatives that their popular vote exceeded the Liberal-NDP vote in just two elections over that time period.
Now, that's not to say they'd only be in government for 8 years out of every 60. You could argue that a Conservative-NDP coalition would have been the logical end result in 2006. Or maybe they could try to go at it solo in a minority or form a grand coalition every now and then. But if Conservatives hate the compromises they've had to make now, it seems odd that they'd give away any chance of ever forming a majority government and condemn themselves to perpetual deal-making with the socialists and tree huggers.
NDP supporter Robert McCelland does prescribe a motive for Harper:
Harper’s coalition of reformers and red tories is fracturing once again. Proportional representation would allow these two factions to separate and pursue their own agendas without losing a disproportionate number of seats like they would under the current first past the post system. The right half of the political spectrum would essentially then have the same dynamic as the left with the red tories battling the liberals in the centre while the reformers pulled to the right in the same way the new democrats now pull to the left. Without the reformer baggage the red tories would then have a far greater chance of pulling support over to them from the liberals.
And that is probably what would happen eventually in a PR system. But you need to ask yourself this: If Stephen Harper's legacy is that he was the man who united the Conservative movement in Canada, why, oh why, would he want to also be the man who oversees the destruction of that party? Especially when you consider the PC/Ref/CA vote never matched the Liberal vote in three previous attempts, never mind the Liberal/NDP vote.