Actually, in June of this year the USDA clarified the Lacey Act to allow musicians *carrying instruments as part of their personal baggage* to re-enter the country with the guitar without furnishing paperwork.
This does not apply, however, to guitars that are being shipped from outside the US into (or back into) the US, nor for guitars shipped from the US to other countries. The problem is not generally shipping OUT of anywhere (US included), it's entering the country it's bound for and how diligent (or bored) the Customs agent happens to be. They may look at every single guitar. They may not look at any. It's a crap shoot.
It is true that instruments past a certain age would be grandfathered and okay to import, yet the burden is still on the shipper to document the instrument's age to the Feds. Even a Martin guitar from 1926 with Brazilian rosewood back and sides could be at risk if you can't document the build date to the Feds' satisfaction, or, say, prove that any more recent repairs to the instrument did not involve a banned wood. It's anything on the guitar ... shell, a bone nut or saddle, pickguard, etc.
If you don't have the correct paperwork, you should be very careful trying to import any guitar into the US ... and it's the responsibility of the seller (or shipper) to have the correct documentation, or to warn you that you are taking a risk trying to get it into the US and that the responsibility for the loss would ultimately be yours to bear. Shipping insurance (FedEx, UPS, USPS) will not cover such a loss, and my business insurance won't, either, it's specifically excluded.
I am very upfront with overseas customers about these things. But even as a dealer, with an EIN# and import/export licensing, I've encountered some headaches. Caveat emptor ...