jdMorgan - 2:41 am on Jun 25, 2010 (gmt 0)
The ErrorDocument syntax shown will result in a 302-Found redirect to error404.php, instead of serving the error404.php-generated content along with the desired/required 404-Not Found status. This is a potential "SEO disaster." Remove the protocol and hostname and refer only to a local fielpath to avoid this. See Apache core ErrorDocument documentation.
The redirect to /forum (first rule) does not specify a protocol and hostname, so the default ServerName will be used. If this is the 'other domain' you mention, then you can likely fix the problem by specifying a canonical URL as the redirect target.
In your final rule, consider excluding those filetypes which cannot be generated by the rewrite target scripts. Examples might include image, CSS, and external JS files, document (e.g. .pdf, .xml, and .doc) files, and media files (e.g. .swf, .flv, .avi, etc.). By explicitly matching these filestypes in the first rewritecond and thereby preventing unnecessary 'exists' checks, you may see a dramatic increase in server performance (and possibly disk life as well).
[added] Consider the use of <LimitExcept GET POST> instead of <Limit PUT DELETE>. This closes a hole that leaves several other HTTP methods open without a policy. [/added]