One of the criticisms of the stimulus package that Congress passed and the President signed is that many people will save it or use it to pay off debt. However, the money still "goes" into the economy. Those who save it will be providing capital to financial institutions; they don't put it into a lock box but lend it to others. Those who pay off debt are similarly making more money available to lend.
What the rebate does mean is that the government will have less money to either spend or pay down its debt. That means at some time more taxes will be needed to pay for needed projects or to reduce the debt. In other words, the government is doing what many people did to get us into this mess - living on tomorrow's income.
