I think that while the street sign analogy works really well for things like toolbar icons (Apple still does follow it for some of the toolbars, like the one at the bottom of iChat), the difference with program and file icons in Mac OS X is that they are not street signs. I don't have milliseconds to react to my Navigator icon. Yes, street signs aid in finding what I want quickly. However, I am staring at my Navigator icon the entire time I'm using my computer. The Dock puts all those photorealistic icons in my face almost all the time. Icons on my desktop are always there. I do agree that simplistic street-sign-like icons would aid me in finding my programs faster, but I would get sick of them pretty quickly. The current icons are more aesthetically pleasing, and for the most part apple has made attempts to differentiate the icons shape and color-wise as much as possible. The majority of the icons sitting in my Dock have very different outlines and different main colors.
I think designing interfaces requires walking the line between what is the absolute easiest to use and what is pretty. Obviously it's very easy to fall too far into the latter, but let's not fall too far into the former either. I wouldn't want street signs in my Dock.