Marshall - 5:12 pm on Sep 3, 2012 (gmt 0)
If your site has properly designed navigation there is absolutely no need for a sitemap.
If you are at all concerned about accessibility, sitemaps are important. Many styles of dynamic navigation are not accessibility friendly, e.g. easily read by text readers for the visually impaired. Another consideration is that it provides a backup in the event your regular dynamic navigation fails. Remember, not everything works equally well in every browser, or for that matter device. Besides, it cannot hurt.