aristotle - 1:50 am on Dec 27, 2011 (gmt 0)
Whether you should link to it from every page depends on what you're trying to accomplish with it. If it's mainly for users, then you should probably link to it from every page, except perhaps from pages that don't get much traffic.. If it's mainly for SEO purposes, you can use it to redistribute additional PR to the pages that you want to promote. If I understand Tedster's example correctly, it's a compromise that combines both purposes.
I don't think it matters much whether the site map page gets indexed by Google, since it probably won't get but a trickle of traffic even if it is.