The greatest differentiator between SEO success is the person who sets up and administers the site, not the underlying technology. There are plenty of DNN sites which are number 1 in the search results for their specific category.
As Tom has posted, many people belive that CMS based sites are at a disadvantage somehow. This is untrue, and probably based on thinking 5 years ago when the early open source CMS tools weren't particularly SEO focused. DNN emits pure Html back to the browser, just the same as any php solution or set of html files sitting on a server. It's how you setup that html and promote it that makes the difference, not the tool that makes it available to the world.
What you will get with DNN is continual improvements in the field, and a lively market in third-party modules designed to give you control over aspects of your website.
-Bruce