From my experimentation it appears to only affect user using Microsoft’s Internet Explorer browser. I use Firefox and never saw this issue; also tried Google Chrome and no issues there as well.
There is very little info to go on, it took me quite a while to put the piece together as to what is happening. Right now, I am in the process of figuring out a response and have disabled the forum area. It seems to have resulted from a security breach at my host company, Dreamhost.
As I am downloading the full content of the forum for analysis, Windows Security Essentials detected a backdoor virus in the Avatar folder (…and promptly deleted without giving me more info – arghhh).
I am sorry, especially to those who have been affected directly.
I am in contact with Dreamhost and phpBB to figure out a solution and I will hopefully be back online shortly. – Martin, April 25, 2012, 10:00hrs EST