Sitemaps and tag clouds help SEO if done properly but not having them is not an issue. If you are concern with SEO then you need to optimize other things such as having relevant content, adding alt tags and title tags for images, also optimize internal links, optimize your menus to give better crawlability, get back links from relevant external websites etc...
As an example for internal links, if you link from the home page to an article, instead of a link saying "Read more..." or "Click here for more..." you better include keywords "Optimize for SEO without sitemap" and make the link visible to the visitor.
SEO is a lot of little optimisations that add up, it's not only sitemaps and tag clouds.
