I wasn't going to say anything because normally it turns out to be something stupid I've done, but yes, it is looking the same as Retched's post in Firefox for me.
I did a forum upgrade, and the datastore cache wasn't refreshed - those of you who had the old CSS file stored locally, things looked pretty. Those who didn't...not so much.
Thats the problem with CSS. Each browser renders it differently so to have a site thats looks exactly the same on all browsers is easier said then done. Basically just looks like when your browser tried to get he CSS file it wasn't there or timed out. That just looks like straight HTML with no CSS.