Does the current generation of Sitemap Generators follow a link even if it has the NoFollow attribute? Does this affect internal NoFollow links? Does this affect external NoFollow links?
The behavior of a sitemap generator would be specific to that particular piece of software (be it a script or standalone application). I suspect some are able to understand the NoFollow attribute while some do not. Is there a specific sitemap generator you're using?
A1 Sitemap Generator defaults to obey meta noindex, meta nofollow, a nofollow and robots.txt. See this FAQ item about nofollow and noindex.