Wow. The AOL browser defaults the text sizing that large? Bit much, but hey. Also a little annoyed that one of the nags we had from IE for awhile showed up on there with the left side box sticking out past its border :/
In his screen shot, it looks like the stats block was jutting out past it's borders and the forum block wasn't starting until below that whole left set of blocks, so maybe AOL's version of IE decided that, at that size, the left block simply preceded the center block. *shrug*
After a quick glance at the screenshot, I can say that the "Stats" section is what caused this. Particularly the "Posts" and "Newest Member" rows. Rather than clip offending text, IE tends to grow an area to fit, but doesn't update the surrounding areas. For flowing layouts that attempt to use all available space, this can have some unintended effects. In this case, the left and right bars float to either side, leaving the third area (middle) to clear the lefthand bar when it doesn't have enough room.
Haven't used opera in a few years, but Mozilla based browsers wouldn't have this problem. I've run across a couple solutions for this, but they tend to be non-standard IE centric like dynamic CSS attributes.