Former Bush administration official Keith Hennessey has a characteristically smart and well-done post about the downside of government infrastructure investment, and about the flaws in some of the arguments that the Obama administration makes in favor of it:
If you advocate raising capital taxes (on capital gains and dividends, for instance, as Senate Democrats appear poised to do) at the same time you argue for increased government capital spending, you're shifting capital investment from the private sector to the public sector. That will slow long-run economic growth rather than increase it.