This question generally doesn't apply only to Adsense, but all PPC revenue... Everything else being equal, does a click from a US computer generate more revenue for the publisher than say, for example, a Canadian click?
I'd say yes, because advertizers competition will be higher within the US, the prices will go higher.
From my personal experience -Yes. I have two sites with similar subject.Site with mostly US visitors brings higher pay per click then the other , Canadian targeted .
The difference between USA and Canada will not be much. But clicks coming from some Asian countries are horrible.