okay after installing then uninstalling the flash plugin, japanese ie6 now works as it should!! The fact then that there's nothing wrong with the flash-detection code is mostly bad news, as I'll have no way to fix it.
I had setup 4 machines from scratch yesterday..2 eng, 2 jp..the english ones worked as expected but both japanese ones didn't (they were asking to install flash...ARGH)! I should probably test this out on a couple more fresh machines though to see if it really is consistent.
To make it a lot worse, for some reason when the flash script isn't working my entire navigation (which is being called from as external .js and isn't using flash) doesn't show up. But it's somehow related to the flash problem because if you answer yes to install the plugin the .js loads & runs--if you say no it doesn't! But javascript IS definitely working all along otherwise it wouldn't have run the document.write that puts the plugin download request code into the page.
I think I'm gonna go finish ripping my scalp out now..