If you wish to obtain a FAA PPL issued on the basis of a foriegn (JAA/CAA) licence you have to go through the validation process described in both the CAA and FAA websites.
If you already have a FAA PPL, however it was obtained, and wish to add a rating to it you do not need to complete any of the validation paperwork.
Riverside Flight Center has successfully completed IR training for European pilots with "restricted" PPLs post 911 and had no security or any other problems whatsoever.