Pushing Pixels: About those vector icons.
"Why can’t we just use vector-based icons in our application? This question, in a variety of reinterpretations, keeps on resurfacing in various forums, blogs and articles. It is asked in the context of desktop applications where the same icon is used in different ways, starting from very small 16*16 icons in file lists, up to 128*128 icons used on the dock and all the way to 512*512 icons that mostly serve to highlight the artistic capabilities of icon designers, at least until the computing world is taken over by 1200dpi desktop displays."
Kirill Grouchnikov gives a great overview on how and why things are done in the world of icons today.