John Naughton wrote a good summary of the Mythical Man-Month problem (the belief that adding more people to a project will get it done faster) in Why big IT projects always go wrong:

Man-months are a hopeless metric for assessing the size of a complex software project. Why? Basically because a big software project involves two kinds of work: the actual writing of computer code; and co-ordinating the work of the dozens — or maybe hundreds — of programmers working on different parts of the overall system. Co-ordination represents an essential but unproductive overhead: and the more programmers you have, the bigger that overhead becomes. Hence Brooks’s law: adding manpower to a late software project makes it later.

I’ve yet to see a large project where this law doesn’t apply.