Do you happen to be using iPhone/iPod Touch/iPad?
EDIT - of course, if you are, you won't be able to reply, will you (slaps forehead).
If you're using one of the above, the rich text editor doesn't work. However, below it there's a link saying 'Disable rich-text' - if you click on that it turns off the rich text editor (funnily enough) and you can then use it. You're stuck without bold and italics and useful linking tools, but you can still enter those links using html if you know what youre doing*
*if you don't, I might do a 'plain text html linking and image inserting and stuff like that' primer...