When attempting to upload media files, I got a very cryptic IO error message. This occurred in both Firefox 4 Beta and Chrome.
I realised I had forgotten to create the blogs.dir folder and set the permissions, but even after doing so, it still didn’t work.
Interestingly, it worked just fine in IE. The solution for the other 2 browsers is to use the browser uploader. Not sure why the flash one doesn’t work properly.