I'm running Firefox3.0b5 in safe mode, thinking that it might be my webdeveloper bar or something like that, but the forum still has this weird red background and no-one's said anything! I'm a little bit paranoid because I also have something weird going on on Flickr (three of my thumbnails - the square ones - are showing up as a different picture) which also started last night.
I've checked my processes (using procexp) but can't see anything unusual going on - but perhaps this forum is now supposed to be red?
Ah - the penny dropped as soon as I posted this - I cleared my cache (which does not happen automatically in Firefox safe mode) and everything is back to normal on both sites.
That was strange though!