By rights, you can be denied boarding if you don't hold a valid ESTA.
Only a customs form is done on board now.
.[/QUOT
You WILL be denied boarding if you don't have an ESTA
Technically they will let you do an ESTA at check in / at boarding if you get instant approval, but very risky.
Be careful doing an ESTA to. I saw a couple denied boarding on my last trip, as they had completed the form as UK - Diplomat (no idea why) or something, rather than UK - British Citizen !