So fair enough the Americans want to control access to their country, that’s reasonable and as British we’re on the Visa Waiver Program (VWP) again, OK, much less hassle but when they introduced the ESTA (Electronic System for Travel Authorisation) I thought those days of filling our green forms on planes was over. Oh no. You actually need to do both at the moment! This taken from one of their websites:

Q: If a VWP traveler has received ESTA approval, does he or she also need to fill out an I-94W? A: ESTA will allow DHS to eventually eliminate the requirement that VWP travelers complete a paper Form I-94W prior to being admitted to the United States. While DHS continues to work together with the airlines to fully automate the I-94W process, all VWP travelers applying for admission at a U.S. port of entry, including VWP travelers who have obtained travel authorization via ESTA, must present a completed Form I-94W to U.S. Customs and Border Protection. </blockquote> So basically they want it to be electronic but at the moment it's not fully done. So do you need to do a green form if you have a ESTA, yes. At least the ESTA is valid for 2 years and you can update it with where you're going but it's still a bit of a faf.