All I did on one of my WP sites, where the WP files were in /wordpress, was to put /wordpress/index.php into the root, and delete the existing index.html. Maybe some installations need a more complex treatment, but this worked fine for me. Oh, and you also have to change the Settings/General URLs. I have my Wordpress URL as domainname.com/wordpress and the blog address as domainname.com. No trailing slashes.
http://codex.wordpress.org/Giving_Wo..._Own_Directory
This says you have to move the WP .htaccess file into root, but when I did that, I got errors.