I spoke with Andrew Nacin of WP back in July about reversing multi-site from WP 3.0.
Here is what he said:
"Remove the constants, delete the htaccess and let WP create a fresh single-site one, remove the extra tables, remove the two extra fields added to the users table. In fact, toggling the constant is enough in most cases (for testing anyway)."
If you have the old MU installed I'm not sure how much it differs from 3.0MS so you'd be on your own in that case.
If it's a fresh install why don't you just clear the DB and re install from scratch?