This will increase the running costs to US based pools, people will move their mining overseas. If that got targeted (it would be a nightmare to enforce, but for arguments sake...), p2pool would suddenly become more popular. p2pool has already experienced an effect like this, when the big pools experienced mass DOS attacks a month or two ago. People just switched to p2pool so they could keep mining without getting harassed.
And most Americans will increasingly get pushed out/excluded from pooled mining and the slide of the US into increasing technological/financial irrelevance will continue.