The reason why is because we are migrating over thousands of html files over to a new site design and I have setup asp includes for the top and bottom of the page. And marketing is worried that somebody out there will have a bookmark to a page and will get an error if its now .asp. And yes I have told them about custom 404 error message pages.
You could always just write a little vbscript to rename every .html or .htm file on your website to .asp. Just a few lines of code actually.<BR><BR>At least this way, you don't have to force IIS to process the page differently like is suggested in the other post.
You might want to throw out a page with a redirect that tells customers to rebookmark the new page as well, rather than trying to account for the conversion. Or perhaps at least throw up a pop up of some sort.