Didn't England do this? I think Australia is working on it also if they haven't passed it already.
they are working on it

In Australia is that you must have a legitimate reason for carrying one - which includes needing it for work but NOT for self defence. Now, I'd argue that a pocket knife is useful for most jobs, but that's another story - if a cop decided you look dodgy the onus would be on you to prove your legitimate reason - I don't know if "I open my mail with it" would be good enough - there is nothing in the regulations about how you must use the knife at work for it to be acceptable.
Sadly, the best advice is "don't carry anything that looks mean!" (so the 3" length rule is worth sticking to, even though there is no mention of length in the regulations) - and don't be caught in the wrong place at the wrong time!
Flick knives, Balisongs, and Daggers, which are banned everywhere and considered the spawn of Satan are, in all honesty, only banned because the bad guys use them in movies! They are no more dangerous or sinister than any other knife - but it's the perception.
You CAN possess the above 'illegal blades' if you have a permit for the importation from your States Police Commissioner or State's Firearms Registry who administer these permits.