Under the 1958 federal Switchblade Act, switchblade knives are not importable into the United States, and may not be shipped across state lines. On May 21, U.S. Customs & Border Protection (CBP) proposed the revocation of four previous Ruling Letters; the effect would be a drastic expansion of the definition of non-importable knives. The organization KnifeRights warns that revocations would outlaw approximately 80 percent of the current market in folding knives.
The federal law does not apply to the mere possession or carrying of knives, but as KnifeRights explains, many state and local bans on possession or carrying are parasitic on the federal definition. Accordingly, if the proposed Customs change goes into effect, many millions of people who own or carry pocketknives would instantly be defined as criminals.
The National Rifle Association and other Second Amendment groups have issued alerts about the proposed change, but the KnifeRights website is the key source for detailed information. Founded in 2006, KnifeRights is still a fledgling organization; they do good work, and I am pleased to be a member.
[Note: I added text to clarify that the 80% figure is for folding knives.]