For 1.0.2 and earlier, change wp-admin/options-permalink.php. For nightlies, change the rewrite_rules() function in wp-includes/functions.php.
Make sure you move that rewrite rule to the end of the list in .htaccess. It is greedy and will gobble up everything. That is why we have /category/(.*), /author/(.*), /feed/(.*), /comments/feed/(.*), and so forth. If we tried to dangle them all off the root, they would clash horribly and only the first RewriteRule would work.