Sorry, if this has been discussed earlier, but searching for 0.0.0.0 didn't give any results.
I just found something interesting on the Who's online:
Guest (0.0.0.0) 11:17:32 Viewing the board General Discussions.
Now, it's very hard to imagine that someone with that IP can successfully connect to my server.
Or maybe, it's THE l33t h4x0r we all fear? :)
I haven't seen this before, but I must say that I check Who's online rarely, if ever.
Edit: He is still there, after refreshing the page several times, but he didn't do anything, because the timestamp didn't change.
This is nothing to worry about.
Do a search as this has been posted before.
http://www.simplemachines.org/community/index.php?topic=22983.0