Walker,
If you aleady have an FAA cert issued off your PPL then you are ok to go flying in the US, however if you want to undertake any training then that a different matter.
Last I heard was that for JAA training in the US, you require a visa which must be applied for before you leave the UK and takes about 8 weeks. You cant pop done the embassey once over there!
FAA training is a problem, apparently FTOs have been told that they are not allowed to add any ratings to an FAA PPL issued on basis of a foreign licence. You can however still get endorsements in your logbook it seems (i.e. Complex endorsement), as they are not an addition to the licence they are just a sign off in your logbook by the instructor.
This info is about 2wks old so fairly current. Anyone Stateside give an update?
Julian.