Lke Robert said above, the problem with another round of shipping out checks to the people is that the people have quit spending and are instead saving and/or paying down debt.  While it is good in many ways that people are behaving more repsonsibly, saving money and paying down debt does nothing to create or save jobs.  We already saw that the last round of stimulus checks did little for consumer spending, and those were before the worst of the financial crisis, which really scared people away from spending.  So that's what the infrastructure component of the stimulus is most aimed at-- jobs.  Obama's experts are telling him that the stimulus needs to be targeted so that the money is actually spent, not saved or used to pay down debt.

As far as the payroll tax holiday, I suspect that Obama's economic team would be worried that businesses would behave much as scared consumers have-- saving the extra money or paying down debt rather than spending.  Look at the banks for instance-- the government parceled out TARP money to banks with the hope that the money would be lent out, but instead, the banks have just sat on the money.