How this is supposed to fix the economy is, gosh, magic. Either the money was already in the economy, confiscated from the productive sectors by the government to be given to the poor (to a straight-up Marxist redistributionist scheme termed a "tax rebate" so as not to fret conservatives), or it will be produced as if out of thin air.
The real mystery is why the Republicans went for it. The Bush tax cuts of 2001 and 2003 are set to expire in 2010. This means that massive tax increases are coming in a couple of years if nothing is done. Why, in a time of economic uncertainty, during an election year no less, would the Republicans not make a campaign issue out of these impending tax increases?
The economy may seem wobbly now; how could it handle the shock of the unaverted tax increases? Even now, economic expectations are being adjusted downward according to their increasing likelihood. It certainly doesn't seem to be a favorable climate for defending tax increases.
Rather than acquiescing to the stimulus sophistries, the Republicans could have forced poll-watching Democrats to choose between possibly keeping their coveted tax increases or not angering voters. Their eager refusal to do so is vexing, to say the least.