Well, I see three possibilities, the last of which is highly unlikely, the first possibility is where my money is.
The most likely, IMHO, is that some type of plugin or spyware has infected your browser. If you don't have it grab Spybot at
www.safer-networking.org/ and run a check on your system and immunize your browsers,(browser must be closed during immunization). Browser attacks are made possible mainly with scripting such as javascript which has your browser executing code from the sites you visit. The web bug could have been picked up anywhere and not necessarily here.
It is slightly possible that this is some type of redirect from your ISP but I highly doubt it. If your ISP wanted to block this site you would get a 403 message. You could try surfing here using the webproxy located at
http://www.hidemyass.com/ which should bypass any block or redirect your ISP has in place.
If anyone had hacked an admin pass and installed any nasty script I would really be surprised and perhaps shocked that all they did was install a web bug that occasionally shows up for only a few members. As a matter of fact I would love to know how such a selective bug of that sort would work. So basically I would rule this out as a possibility.