From a practical standpoint, 999 times out of 1000 you will not have a problem with an open ticket. However, there is always the 1 ........ officer who decides to enforce the letter of the law rather than the spirit.

A lot will depend on the transporting carrier. Some will NOT board you on a flight to the US without a verifiable return/onward ticket if traveling under the VWP. I suggest you check with the carrier first and make sure that they are ok with it, as they will be the ones fined if you are denied entry.

Finally, an easy end-around of this regulation is to book an award itinerary (unticketed) back to your home country for some date within 90 days of entry. Carry a copy of this itin with you, show it to the officer only if specifically requested and cancel it once you have been granted entry.