I would like to rewrite requests to http://www.mysite.com/foo to http://foo.mysite.com without the user's address bar changing. Using IIRF I can do the rewrite, but only if I use the [R] modifier flag which makes the rewrite a redirect. Is there a way for me to transparently rewrite requests to a subdomain?

Here's the rewrite rule I've been testing with:

RewriteRule ^/foo/(.*)?$ http://foo.mysite.com/index.html?$1 [R,L]