According to the FAA's website (www.faa.org) under CFR14 chapter 1 part 61.41 the holder of an ICAO instructor's rating can teach the FAA syllabus providing they do so outside the US. It makes no mention of teaching JAA inside the US. However, since instructing is classed as a commercial activity and you are required to hold a FAA CPL, CFI and relevant work permit to exercise commercial privileges in the US I would think you would be on very dodgy ground if you attempted to use your JAA commercial privileges in their back yard.. .Best to email the FAA for chapter and verse, though...