Paul Krugman and Robert Barro have both responded to Clive. The difference between the two responses is striking. Paul Krugman says, basically, "You can't expect me to dignify my responses to amoral dolts with a measured tone." Robert Barro brings the economics. Quite a lot of it. The technical problem of calculating the multiplier remains in dispute, but he lays out the parameters of the problem, and what you need to know in order to solve it, quite admirably.