Mark Thoma runs through some numbers. Of course the plan has not been announced, so I don’t have a good sense of it. There is talk of five million new jobs, however, and that figure makes me suspicious. There is very often a trade-off between spending the stimulus money well and spending the money to create new jobs. For instance the better "Green Energy" ideas mostly involve hiring or reallocating skilled people who already have jobs. Many of the recently unemployed seem to be coming from construction, retail, finance, and, soon, manufacturing. If we spend to build up mass transit, can we cobble together a team from these people? And do they live in the right regions, have the right skills, and have the right willingness to lay some extra Metro track? Don’t accept the rosy scenario numbers until you see at least partial answers to these questions.
The New Deal put many people to work in the physical construction of infrastructure; at that time the skills of labor were more homogeneous, people were more desperate for work, and it was more expected that "labor" meant physical labor for able-bodied men.
Addendum: Arnold Kling makes a similar point.