So far as I'm aware, if you don't hold any licence or you are an FAA licence holder then you need an FAA medical to fly in the USA BUT if you hold a CAA/JAA licence then you definitely need a UK/JAA medical. An FAA medical is absolutely no use to you. The FAA licence which is issued to you in the USA on the back of your CAA/JAA licence is "Issued on basis of and valid only when accompanied by United Kingdom Pilot Licence Number PP******G/A all limitations and restrictions on the United Kingdom Pilot License apply" - That means if your UK licence aint valid ie: you don't have a current JAA medical then you cant fly in the USA regardless as to whether you have passed the FAA medical or not!