Otto42, I appreciate your oh so annoyed comments very much, and have followed your directions to the T. I maintain about 10 WordPress sites, and when the call came for the upgrade to 2.9, I did so, but some of them went fine, and some of them did not.
For the one that did not (visual editor broke) I have just followed your 2-step instructions. I downloaded fresh 2.9 code, and manually uploaded it to the site where the TinyMCE editor is now failing, flushed my caches, and nothing changed. The TinyMCE editor did NOT repair itself, and you telling me I did it wrong does not change the fact that I did it right, or at least, exactly as you specified. Since it was working before 2.9 came along, and is not working now, the fault is wp 2.9's, not mine.
I have also done the usual uninstalling and reinstalling of all the plugins with fresh code too. This site is now all fresh brand spanking new code, and the TinyMCE editor is still not working, as we've tested it on multiple computers and multiple browsers.
I wish that TinyMCE came as a totally separate plugin that we could activate, deactivate, delete, reinstall with fresh code, & reactivate at will -- without having to replace all of our WordPress code just to do it.
As it stands, however, it looks like unless you come up with a better answer, explanation, and apology for how 2.9 broke our TinyMCE editors, I am replacing it with the FCKeditor plugin, which works perfectly well, in some cases better --- as I've seen on another of my WordPress sites --- and IT came through the 2.9 upgrade just fine.
For the 2.9 bug report -- TinyMCE comes up fine when adding a new page, but refuses to come up when editing some, but not all, existing pages. I've spent enough time on it for one day. If I could tell you more, I'd be on your payroll.
My fix is to replace TinyMCE by installing the FCKeditor pluglin.