Well, this started out as a question and as I wrote it up, I solved it!!!  so I will share.

Environment:

• DNN: 4.3.4
• FCKEditor: 1.00.06
• IIS 6 however '.html' RUNS AS AN ASP.NET FILE
  

I have set up ISS to run .html files as though they were .aspx files.  This is better for most clients as it meant they could keep their old page names (SEO implications) and the general usability of .html is higher then .aspx.

BUT... FCKeditor uses template files named "fckeditor.html" etc. which caused a 404 error.  Since these run .NET, DNN kicks in and looks for the TabID of the page.  Since there isn't one it causes the 404 error.

Solution:

• Open the IIS properties for folder: ~/Providers/HtmlEditorProviders/FckHtmlEditorProvider/FCKeditor
• Set the "Execute Permissions:" to "None"

Now the files won't trigger the DNN rewriter and the editor works as planned. 

It was luck that the image browser template uses the extension .htm so the same problem doesn't exist for those features.  I humbly request this stays this way for future releases.  Better would be if one could set the extension of the templates to something completely neutral.

In that case, you may wish to consider ISAPIrewrite  (http://www.isapirewrite.com - the IIS equivalent to mod_rewrite) as there will be other issues.  ISAPIrewrite runs before DNN so you can hijack URIs.

For instance, Google site verification requires either a meta tag or file with a code in it is always accessible on the server.  IMHO, the meta tag route is messy so the file is the way I go.  Google requires a .html extension so I need the rule:

RewriteRule /google(.*)html /google$1htm [I,L] 

The physical file is a .htm but Google doesn’t know the better. 

I should point out the .htm extension is not run as a .NET file.