If anyone has a reference to an actual law or ordinance contrary to what I didn't find, please post here! The absence of evidence of a law in my cursory search certainly doesn't mean that there isn't one somewhere, so check with your local District Attorney's office or a lawyer (be sure to get an actual law/ordinance cited if they tell you you can't have it).
-RJP
Well I know for sure that after the Hollywood shootout it was PROPOSED to make POSSESSION of a vest a FELONY in NYC. Don't know if in fact it ever passed. Armoring a car in NY requires registration. But seeing that slingshots are also a class A Misdemeanor (1 yr in jail 10K fine) it wouldn't surprise me.
Also poor PhilW in Australia is out of luck...

update:
NY State penal code
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Section 270.20 Unlawful wearing of a body vest
1. A person is guilty of the unlawful wearing of a body vest when acting either alone or with one or more other persons he commits any violent felony offense defined in section 70.02 while possessing a firearm, rifle or shotgun and in the course of and in furtherance of such crime he wears a body vest.
2. For the purposes of this section a "body vest" means a bullet-resistant soft body armor providing, as a minimum standard, the level of protection known as threat level I which shall mean at least seven layers of bullet-resistant material providing protection from three shots of one hundred fifty-eight grain lead ammunition fired from a .38 calibre handgun at a velocity of eight hundred fifty feet per second.
The unlawful wearing of a body vest is a class E felony.
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So now I am thinking that an aggressive DA would want to tag this on to a home owner who shoots an invader.
Yuk.... please beam me up Scotty.