Hi Tim,
If you are going over to the States to pursue a course of training, for say an FAA IR then you will need the letter of authenticity before the Designated Examiner will carry out the checkride, the letter was asked for at the same time as my FAA PPL (61.75 based on my CAA PPL).
However if you're just going to the United States to rent and aeroplane and fly about a bit then you should be fine with your present FAA PPL without the letter. BUT there have been some rumours of some FBO's requesting to see the letter of authenticity with older FAA PPL's issued under 61.75 before renting aircraft to people.
If you are just going to rent, you'll probably be better off asking the Airmen Certification Branch of the FAA at Oklahoma. The contact details can probably be found on the following URL page.
http://registry.faa.gov/airmen.asp
Best wishes,
Charlie Zulu.