I'm a layman on economics but do my best to make sense of the arguments out there. I found Bruce Bartlett helpful as usual:

The problem is that fiscal stimulus needs to be injected right now to counter the liquidity trap. If that were the case, I think we might well get a very high multiplier effect this year. But if much of the stimulus doesn't come online until next year, when we are likely to be past the worst of the slowdown, then crowding out will greatly diminish the effectiveness of the stimulus, just as the critics argue...

Thus the argument really boils down to a question of timing. In the short run, the case for stimulus is overwhelming. But in the longer run, we can't enrich ourselves by borrowing and printing money. That just causes inflation.

The trick is to front-load the stimulus as much as possible while putting in place policies that will tighten both fiscal and monetary policy next year.

How we do that is above my paygrade but the principle makes sense to me. Any big stimulus should focus on getting demand into the economy now or very soon; it should be followed by a pledge to look very closely at the long-term fiscal pressures and alleviate them. Social security and Medicare have to be on the table; but so too do corporate welfare, and defense. Not easy, but in this crisis, opportunity.

[Update: readers quickly noticed that Bruce links to a CBO report that doesn't exist. I've removed the sentence and the link. The broader point remains.]

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