Currently wrong password errors are logged to error.log like this:
2008-01-31 02:07:08: (http_auth.c.1132) digest: auth failed for root wrong password
And this is basically of no use as one cannot see from which IP/host the wrong password attempts came. So if the error.log is full of those then you know someone has mounted a wordbook attack on the server, but one has no idea from where it came.
This should rather be logged like this
2008-01-31 02:07:08: (http_auth.c.1132) digest: auth failed for user root, IP:1.2.3.4/www.malevolent.de, reason: wrong password
Ideally configurable but hardcoded would already be a start. Using such a log it would then also be possible to run automatic blocks via fail2ban or similar script in thee cases.