avrodamo,
I Just heard from Homeland Security in response to your question.
If you already hold a U.S. Licence, whether a Full FAA Licence or a Restricted FAA Licence issued on the basis of your Foreign Licence, you can enter the country on a Visa Waiver as long as it is just flying for pleasure. You cannot take any instruction or work on any ratings other than the aircraft checkout for insurance purposes.
Happy Flying,
Capt. Richard J. Gentil, Pres.
Naples Air Center, Inc.