Please upgrade your SMF forum to SMF 1.1.9 or SMF 2.0 RC1-1. There has been a huge string of hacks going around. You can read about it here:
http://www.simplemachines.org/community/index.php?topic=309717.20
You can get the upgrade by going into your admin and clicking on the link. You can read about it here:
http://www.simplemachines.org/community/index.php?topic=311899.0