Here's the deal as I see if. If the Liberals introduce a fall budget that is unsupportable it does not necessarily lead to a fall election.
As we saw in the spring it takes a long time for a budget bill to get through the House. The opposition parties can use this to their advantage.
Assume along with me (and wait for it, this is a HUGE assumption) that the budget will be opposed by the NDP as well as the Conservatives (the Bloq will oppose anything the Liberals propose) AND the NDP can be trusted to work with to block the budget. Instead of voting against the budget all the opposition parties can abstain from the vote and obstruct the bill in committee. If this works you can stretch things out until at least the preliminary report from Justice Gomery.
If the Liberals look like they might invoke closure on the debate then one of oppossition parties could preemptively use a supply day motion to prorogue the House (I would love to see the Liberals spin this into a confidence vote after what they did in the spring). If this all fails let the government fall and go to the Governor-General, play rock-paper-scissors to pick a winner between Stephen Harper and Jack Layton, allow them to be a lame-duck Prime Minister until February and have an election "30 after Gomery".
Hey, you never know? After this spring anything is possible.
