If you decide to do this core please don't include those useless HTML sitemaps that some developers pass off like a sitemap a search engine will really find useful. The only useful sitemap to a search engine is a XML sitemap, anything else is just wasting time and resources.
Although one may not help your rankings it makes it easier for search engines to find new stuff to index and IMHO get a link listed allot faster than traditional crawling alone.
To those who supporting argument is because the sitemap would be for the forum only, well have you heard of sitemap linking? You can have a main sitemap which contents contain other sitemaps. If sitemap linking is not for you did you know you can specify multiple sitemaps in your robots.txt?
So if I had to do a supporting argument I can compare it to RSS feeds which I don't use and SMF already has, so why not a XML sitemap.