mod_rewrite condition and rule effects POST vars in a bad way - mod-rewrite

Question

I have 2 domains mydomain.com and mydomain.org. The site lives at mydomain.org so I want any attempt to mydomain.com to resolve to mydomain.org.
The following mod_rewrite rule that works to a degree.
RewriteEngine on
RewriteCond %{HTTP_HOST} ^mydomain\.com$
RewriteRule ^ http://mydomain.org%{REQUEST_URI} [L,R=301]
After I implemented it and tested it I felt it was doing everything I needed until I submitted a form with method="post".
For some reason, this mod_rewrite trashes my _POST vars
I am working solely from mydomain.org (which is the TLD I want the site to resolve from and where I submitted the form from).
Does anyone know of an adjustment to my condition and rule to not lose the _POST vars?
I identified something interesting. I plugged-in the HTTP Live Headers add-on in Firefox. When I use the mod_rewrite I get an "HTTP/1.1 404 Not Found" and when I turn off mod_rewrite I get a "HTTP/1.1 200 OK". The same page and PHP code behind is used. Again, when I have the mod_rewrite directives turned off, the _POST data comes through. When I turn on the mod_rewrite directives, the _POST data does not come through.
MOD_REWRITE Turned Off:
http://dashausmuseum.org/subscribe.html
POST /subscribe.html HTTP/1.1
Host: dashausmuseum.org
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:5.0) Gecko/20100101 Firefox/5.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip, deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Connection: keep-alive
Referer: http://dashausmuseum.org/subscribe.html
Cookie: __utma=74430599.461726749.1312575846.1312897084.1312899646.5; __utmz=74430599.1312575846.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __utma=1.480711209.1312899756.1312979975.1312981669.5; __utmz=1.1312981669.5.5.utmcsr=dashausmuseum.com|utmccn=(referral)|utmcmd=referral|utmcct=/directions.html; __utmb=1.9.10.1312981669; PHPSESSID=7f4a74d7fde56cf901aa85511410b7f6; __utmc=1
Content-Type: application/x-www-form-urlencoded
Content-Length: 60
email=abc&firstName=&lastName=&address=&phone=&submit=Submit
HTTP/1.1 200 OK
Date: Wed, 10 Aug 2011 13:18:31 GMT
Server: Apache/2.2.17 (Unix) mod_ssl/2.2.17 OpenSSL/0.9.8e-fips-rhel5 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 PHP/5.3.4
X-Powered-By: PHP/5.3.4
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Content-Length: 5420
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: text/html
MOD_REWRITE Turned On:
http://dashausmuseum.org/subscribe.html
POST /subscribe.html HTTP/1.1
Host: dashausmuseum.org
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:5.0) Gecko/20100101 Firefox/5.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip, deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Connection: keep-alive
Referer: http://dashausmuseum.org/subscribe.html
Cookie: __utma=74430599.461726749.1312575846.1312897084.1312899646.5; __utmz=74430599.1312575846.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __utma=1.480711209.1312899756.1312979975.1312981669.5; __utmz=1.1312981669.5.5.utmcsr=dashausmuseum.com|utmccn=(referral)|utmcmd=referral|utmcct=/directions.html; __utmb=1.10.10.1312981669; PHPSESSID=7f4a74d7fde56cf901aa85511410b7f6; __utmc=1
Content-Type: application/x-www-form-urlencoded
Content-Length: 60
email=xyz&firstName=&lastName=&address=&phone=&submit=Submit
HTTP/1.1 404 Not Found
Date: Wed, 10 Aug 2011 13:20:32 GMT
Server: Apache/2.2.17 (Unix) mod_ssl/2.2.17 OpenSSL/0.9.8e-fips-rhel5 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 PHP/5.3.4
X-Powered-By: PHP/5.3.4
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Content-Length: 5329
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: text/html
Does anyone see anything in the HTTP Headers that gives a clue as to what is going on?

Answer 1

Something else should be wrong, rewrite rules can't destroy _POST vars.
How are you setting/calling them in your code?

Answer 2

There is some discussion about what should happen to a POST transaction in response to a 301 (or other) redirect. Here is one example where the author suggests it is "messy".
Is it possible your browser is converting the POST to a GET request? Is it also possible the behavior is different in different browsers?