I am going hour building in June and the US embassy recommended getting a B-2 visa. Now I agree that technically you don't need one, but FFF is absolutely right that they can (and have) turned people away for no reason other than they don't understand or believe you.
I paid £62 for it (it is valid until 2013!) and it only took 9 days from first picking up the phone to getting the passport back.
You can just keep your mouth shut but if they ask you straight out if you're going to do any flying (having seen the headset, fuel tester and flying suit when they x-ray your luggage!) then you are going to get into the sort of complicated conversation that £62 could prevent.
Hufty