First, I've got to say the responses that have been posted when people have tried to report this have been poor. The responses have just been a simple repetition of the same "solutions" which have been nothing of the sort.
Second, there's clearly a whole class of problems here and I'm focussed on the one I have for obvious reasons.
So, how to verify you've the same problem. Use some sort of debugger tool like firebug etc, Peter Hengl gives a nice description here : http://wordpress.org/support/topic/164990
The first thing to establish is that it's not a cache problem, at least not in the way that keeps getting posted. For a start, browser and network caches don't work like that. What I did find was that different files were returned using different clients, any browser (opera, safari, firefox) over three OS'es (Mac OSX, XP, Linux) returned a "cached" file that didn't reflect debugging changes I made on the server, it was instantly obvious from a network trace that this wasn't held in any browser cache. However, curl and wget instantly reflected the debug code, someone should have tried that before blaming caching as it's obvious when you look at the code in tiny_mce_config.php why that's going to be different. In fact, looking at the code I'd guess this would also work correctly (this far) in IE5.
To fix this, remove the files from wp-content/uploads/js_cache and you'll start getting an up to date copy of the mce code. NOTE, not everyone will have this issue as such, the cache code is broken in a way I've not fully analysed so this is a nuisance rather than the root cause.
Next, to the source of the real problem. The debugger instantly spots this, it's related to some I18n code and follows back to the get_locale function. The function is weak and used weakly in places which is why there's a problem. This problem will most likely effect people not using 100% English language environments like Peter who's German or me with Swiss/French locale, in fact, Peter's nationality already made me guess the I18n code was broken.
The simple fix is to go to your wp-config file and set the WPLANG variable to something suitable. For me, "en" is fine.