Forum Moderators: phranque
The first thing I noticed it that you did not escape the period in regular expression. Certain characters are reserved for regex pattern matching and they must be escaped if you want to match them literally.
\.html
Then next problem is the fact that RewriteRule doesn't "see" the query string part of the URL. You will need to use RewriteCond to catch the query string.
RewriteCond %{QUERY_STRING} ^(.*)$
RewriteRule ^(.*)\.html$ index.php?arg=$1&%1 [L]
The rule above just pins the old query string to the new URL, since it seems that's all you wanted to do.
Be sure to check out the Apache Library [webmasterworld.com] for some good references.