I have two of them :

#1. I want to rewrite the mime-type for any files with the extension of .xhtml or .xml to text/html IF the user agent contains an instance of "MSIE":

RewriteEngine on
RewriteRule \.(xml|xhtml)$ %{REQUEST_URI} [T=text/html]

does not seem to work for me. (I'm using Apache2 for Windows right now, can't boot into Linux to check it out this moment)

I've also tried REQUEST_FILENAME with no luck.

#2. Is it possible to look at last modified time stamps for a requested file, and if it is say more than a week old, issue a redirect elsewhere? I think I may need to use PHP or something instead, but if I could contain this solely to .htaccess, that would rock.