Here's a useful categorisation of project types from Frank Patrick (on the agileprojectmangement yahoo group):
I usually describe projects as falling into one of two types...Olympic Stadium projects and Oil Well projects. One has a solid drop dead date, the beating of which adds little additional value (other than maybe easier sleeping before the opening ceremonies), and the other is one in which the completion turns on a cash register, so every day sooner provides an additional day's value. I suppose the agile concept of adding incremental value along the way to final completion is more related to oil wells than Olympic stadia. (Although, for a roofed stadium, I suppose making sure the field and seats are ready might be more valuable than bolting on the roof.)
He's got more detail here.
