...and don't forget that you need a visa (B-1 for example) to undergo training in the US. Entry under the Visa Waiver Program is intended for visitors (tourism) only, not for training!
Some people on this forum are still under the impression, that just because they got in and out (and got away with it), it is legal - it's not:
Extensive coverage here:
http://www.pprune.org/biz-jets-ag-fl...highlight=visa