I usually carry a 17"-bladed watermelon knife, but its ALWAYS kept in a scabbard strapped to my motorbike. No problems so far.
Seriously, though, I once asked the salesman in a cutlers about legal knives (he had some of those fancy 8" damascus steel stilettos and huge foot-long bowie knives) and he said they're all legal. Then again, he was also selling powerful crossbows, blowpipes, etc., and I'm pretty sure they're not legal, so go figure.
I expect it depends where you are -- I've gone through police checkpoints while going hiking, carrying a machete that looks like a sword, with no problems. I'd imagine the cops wouldn't like it much if they found me carrying it around in Dinghao, though.