It is most likely an issue with how Opera renders tables. The sidebar that Infini made for the post display uses embedded tables and magic to somehow force the table heights to be 100% of their parent element even though their parent element is determined by variable height. When working on an SMF 2.0 upgrade I used a different method involving relative/absolute positioning to get the sidebar look. I PM'ed you a link to the goodness; I actually didn't test it in Opera but I think I did good.