This US visa waiver system really is about as complicated as a visa itself in terms of use, even if we do avoid the whole application hassles.
I came into US by land from Canada and flew out from Boston direct to UK. Not sure if this played a part, although can't see how.
Wasn't clear about who was meant to pull out the green slip, as only encountered one DHS official going airside and he seemed more interested in my boarding pass than my passport.
Think I'm going to check with airline....