Just thinking aloud, analyzing some alternative scenarios. I must admit, this showdown is pretty entertaining to a guy like me, if nothing else.
What could possibly happen?
1. Conservatives back down. So then, what would they propose? IF they accept ideas from the opposition, they can always blame a failure of the stimulus on the opposition in the next election. "Hey, it's not our fault, it was their idea!" "Look at deficit resulting from their spending!" However, this would be risky for Harper to sell to his own party, which already might be losing patience with him after their convention. I doubt they'll back down unless they think a coalition is a real possibility, since I think the Conservatives have more to gain in a premature election - possibly a majority.
2. Liberals back down. Unlikely, when they could do #3 below.
3. Enough Liberals abstain to pass the update. I think this could happen, but it seems a pretty risky move to me. In the last election the BQ and NDP pushed the fact the Liberals had supported Harper so much already. And the (lack of any) plan makes so little sense. Even though the Liberals would vote again, if they don't kill it it'll be spun as support for the Conservatives.
4. The motion is amended by the opposition. Is this possible? Could the opposition hijack the bill and amend it, then pass it? If this is possible, the Conservatives are in a minority, they couldn't even kill their own bill... but they could withdraw it. The Conservatives could blame the amendments later if things go wrong.
5. The motion is defeated as a matter of confidence. Then what? This is certainly the most entertaining possibility.
5a. An election is called. Each side would blame the other for failure of the government at a time of economic crisis. I think the only people that want an election now are the Conservatives, thinking they could get a majority. They can't call an election themselves - it would be suicide. Unfortunately, I speculate such an election would end in a Conservative minority - AGAIN.
5b. A coalition forms. The Conservatives' roar of undemocratic behaviour would be deafening, completely ignoring that they were a MINORITY government. I find many Conservative arguments grounded in majority rule taken to the point of majority tyrrany. Emotionally, this appeals to me as poetic justice. There is just way too much to speculate on here, there's more than enough material here for many blog posts. Hmmm...
5c. The governer general simply refuses to call an election. I'm not sure this can happen, I don't think there is a precedent for it.