Static Caching gotcha
We've released the free Scheduled Cache Invalidator addon to help with this.
If you are using full static caching, future-posting entries does not work: when the sitemap is cached, it is cached until a change is made to an entry, taxonomy or is manually flushed.
In other words, if you create a post that won't appear until 'tomorrow', and the XML sitemap is generated, when tomorrow comes around and your new post becomes publicly visible, the XML sitemap won't be updated until the cache is invalidated.
You could achieve this by:
editing content
running a daily command that clears the sitemap cache