The appropriately named MapResources.com has the highest detail vector maps that I've been able to find. Their files are excellent quality, with each element type layered.
Andy M pointed me to this: http://commons.wikimedia.org/wiki/File_talk:BlankMap-World6.svg/Documentation
Somebody needs to convert this to JSON asap :D
It's not a slam dunk, but I know that OmniGraffle has some vectorized stencils.