I am going to replace an old html site with dotnetnuke but htis site is highly visited and has been live for 10+ years. Has a number 4 position in Google among 24 000 000 million hits so I am thinking of if www.thesite.com/special_facts_file.html can run as un dotnetnuke page? Has anyone installed dotnetnuke and having it spages run with the same exact url as normal html files have? If so this looks like my easiest approach. Just replacing the entire site with dotnetnuke pages rendered as if they where html accordinglto the extension. Of course I want them to run like DNN .aspx file but looking like .html files. The page rank on this site is very high and also on many of its html pages inside the site.

hmmm I am a bit scared and do not want anything to go wrong and drop every value of pagerank this guy has. Therefore I am looking for a safe approach SEO wise..... I hear you but I still think I'd lik eto try to map .html to be rendered in IIS. I have full control of IIS because of our own servers

Then you want to use the IIS URL Rewrite Mpodule if you're using IIS 7.x or ISAPIRewrite on previous cversions of IIS.  You want to add a 301 redirect, that will not change any page rankings.

Jeff