FYI - As of 12th March completion of the online ESTA process has become mandatory
Really hate to disagree Tightslot, but I just got back from yet another EDI/EWR/IAH round trip on Friday (13th) and when clearing immigration at EWR I asked when the green & white I-94 forms would be phased out. The guy on the desk said that they had hoped that it would be last week, but they were still having some issues ( i.e. the system doesn't work) in getting accurate information down to the immigration desks at some of the major gateway airports, not to mention some of th smaller ones. He told me that another trial was due shortly, but it wasn't certain if this was the final fix/rollout.
Got to compliment the guys at EWR & ATL on the immigration desks always polite, helpful & willing to answer questions. Just wish the guys at the major UK gateway airports could match them.