Another reason why mod_rewrite won't work is because a VirtualDocumentRoot is set up. This is maddening because it doesn't overrides the way mod_rewrite rewrites urls. This might be a good setup if you have subfolders such as "subdomain" subdomain.domain.com, but it breaks a mod_rewrite, so refrain from using it.
In Centos 4, check /etc/httpd/conf.d/virtual_hosts.conf
<VirtualHost *:80>
ServerName subdomain.domain.com
VirtualDocumentRoot /var/www/%1
DirectoryIndex index.html index.shtml index.php
</VirtualHost>
If you care to have this working, then check out http://drupal.org/node/20804, they say that "VirtualDocumentRoot causes mod_rewrite to set an incorrect default value for RewriteBase", so you have to set a "RewriteBase /" in all your .htaccess files.
MeasureIt