As far as SEO is concerned, there is nothing wrong with using a file ending with .aspx, .asp, .htm, .html, .php, .cfm, etc.  All search engines will recognize that it is a different file type representing a web page, and you will not be penalized.  However, unless someone knows a trick that I don't, you cannot use .htm or .html extensions without having the ASP.Net worker process render the .htm extension.  While you can do that, this has a huge security drawback, not to mention that is also breaks the FCKeditor.