If you read Mark Thoma's blog, you will have seen his passionate response, called "Modern Macroeconomic Theory and Fiscal Policy" to some self-styled "modern macroeconomic theorist" who claims that people like Brad Delong and Krugman who think that fiscal policy can help us out of a recession when the interest rate is at the zero bound are not modern macrotheorists and so are obviously wrong.
Thoma and Krugman's response has been to cite papers by DSGEers in which a version of the model does allow fiscal policy to work - papers by Eggertson and Woodford and others of that ilk. Read the two comments by Robert Waldman to be reminded of why he- Robert - is a national treasure. Waldman says the DSGEers cannot be presumed to have explained anything about the modern macroeconomy at all, so the idea that we have to reject the idea that fiscal policy is efficacious if a DSGE model can't show it to be is a nonsensical non sequitur.
I've looked at the latest Eggertson paper. They are all -all the fiscal-policy friendly DSGE papers, I mean- alike in making the mechanism by which fiscal policy works in a liquidity trap this: it increases expected inflation. This is not the mechanism by which it works in Keynes. Of course if it works, it increases Y and that will increase inflation and expected inflation chez Phillips/Friedman/Phelps. The expected inflation is consequence, not cause.
The reason that it works in Keynes, of course, is that it increases demand. Period. Put another way: In a liquidity trap, the problem is that the Wicksellian/Keynesian natural rate is below the expected deflation rate (the latter may be a small negative number, as it is now), which latter is the lower bound on the real rate of interest given that nominal rates can't be negative. Fiscal policy in the trap works not, or not necessarily, by reducing the lower bound, but rather by raising the natural rate above the lower bound. But this can't happen in the DSGE models, because the natural rate - what they call the natural rate, which isn't what Keynes and Wicksell meant by it - can never change. That's a fact about their ridiculous models, not a fact about the world.