I recall that I posted a couple months ago that some states meet low risk entry requirements, and everyone posted back that European countries would never parse the USA into smaller subsets of states with high and low risk.
“The hen is the wisest of all the animal creation, because she never cackles until the egg is laid.” --Abe Lincoln
On a serious note, one huge hole in this idea was brought up in the thread Tom mentions--the US has no interior border controls. What would be keeping a New York City resident from driving up after spending some time in, say, Florida, and hopping on a plane to London the next day? Until they get that figured out--or unless Great Britain is willing to turn a blind eye to such things--I don't see how this goes beyond the "talking" phase.