Back after a few weeks off :o)
Exactly the same problems as I've been having... upgrading everything doesn't solve it either (but doesn't do anything bad either).
I've been trying to fix it and here are some things that I've found out:
-It's nothing to do with hosting providers, I've clean installed the latest nightly and tinymce (the text editor) works fine on that ... so I'm guessing that it's something to do with previou installations that have been upgraded
-It's nothing to do with overwriting files in an upgrade... I've clead deleted wp-admin and wp-includes and reuploaded but to no effect
-It's also accompanied by a few other things going missing / a bit askew, for example the 'hide/open' crossesnext to write post have also gone , as has the WordPress badge on the login screen
-It's not browser related
So... what I'm thinking is that this is something to do with older versions that have been upgraded ommitting certain images or functions.
Which is pretty much where I'm stuck!
As are the, um couple of thousand bloggers that I'm hosting!!!
(teach me not to go extending meself like this ;)
Any help / thoughts from people in teh know would really help me sleep better :o)
Cheers, James