g1smd - 5:45 pm on Jul 15, 2010 (gmt 0)
REQUEST_URI looks at only the path, not the domain.
HTTP_HOST looks at the domain name. Use that instead.
The [R=301] flag makes this a redirect. You need a rewrite so replace that with [L] instead.
Your Wordpress rewrite can be optimised quite a bit. There are several prior threads in this forum with exact code details.