See actual comment in Watchdog log

That certainly is an interesting idea, however that would only work on the comment form and not on any of the other forms that it supports out of the box, nor would it work on new forms created by other modules. Spamicide would have to somehow know about the variable in every form or print the whole form into watchdog, and quite a few custom forms and webforms can be quite lengthy. Not a practical proposition IMHO.

This is another version of the watchdog message I actually use:

      watchdog('spamicide', 
        'Trap on %form_id contains: "%spam". Discarding post.',
        array('%form_id' => $form_id, '%spam' => $form_state['values'][$spamicide_field]), 
        WATCHDOG_NOTICE, 
        l('Whois', 'http://www.ip-adress.com/whois/' . ip_address(), array('attributes' => array('target' => '_blank')))
      );

Notice, that this works for both custom and default spamicide field names. Also this variant utilizes watchdog's automatic ip address logging using it's own column and clears redundant information from the description field.

An additional idea could be to implement a variable_get() which replaces the hardcoded whois lookup url. Then you would be able to configure even a local site lookup by the whois module or any other external service you like. It's also nice to see on a map where the spam comes from :)

I think this is a good idea in general, however since it's supposed to add to any kind of form in Drupal, not just comments. I would have to come up with a way to store all of the submission, and not just the one field, that could be a very large amount of data to try to stuff in the watchdog logs.