I've noticed the following showing up as a page request over the past few days on one of my SMF forums. Best I can tell is it's a spam robot trying to register but getting stuck by the captcha code.
?action=register Result: pictocode could not be decoded;registered (registering only mode is ON);
Since it seems like the spammer isn't successful it may not be that much of an issue, but I'm banning his IP anyway just to be safe.
I also had a really hard time finding anything on the web about this issue, so if you've come here and know anything else that can be done please let us know.
Ryan