There must have been something wrong with the system I think, the highest we've ever had before was about 80. And beyond 100 the server will start trippin, quite possibly collapse under the load. I checked the site logs and there is nothing to indicate such an unbelievable increase at that time.
As for there always being a few guests around, part of that group is made up by users who are viewing the board but not logged in or have been inactive long enough for their session to log out (that's if they're not using cookies).