I'm rehashing a old post of mine about the same topic:
There is a nice article on multi-column lists on A List Apart
http://www.alistapart.com/articles/multicolumnlists/
With simple CSS we can easily float the forum to create multi-column layouts.
I personally don't like them but i can see how they can work on some forums.
Here are few quick and "very dirty" examples made on the fly by editing some CSS with firebug