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.
I think I'm gonna go finish ripping my scalp out now..