So far since applying the registration verification code mode there have been no bogus user registered. Of course, now the error log gets about 60 entries per day but it's easier to clear it than to decide if a user is "real" and manually delete them... I also started deleting all users that even though they had successfully completed the registration process had never posted anything and had not logged back in for over 120 days.
So, the number of users that is displayed at the bottom of the main page should be reletively accurate as far as our membership is concerned.