Being intrigued by this question, and having nothing else to do, I googled a bit more. And buried deep inside the CBP website I found the following:
VWP travelers may arrive on a private aircraft, if:
* The carrier is recognized as a signatory carrier; or
* If they are being readmitted to the United States, within the original 90 day admission period, after a departure to foreign contiguous territory or an adjacent island of 30 days or less.
If a VWP traveler is being readmitted to the United States from contiguous territory or an adjacent island, they will only receive the balance of their original VWP admission period if the traveler is otherwise admissible and meets all the conditions of the VWP with the exception of arrival on a signatory carrier.
From:
http://www.cbp.gov/linkhandler/cgov/...yers_guide.pdf page 20-21. Contains a lot of other useful stuff too. Well worth a read.