The USA men have a lot of success in Davis Cup. I wouldn't count out Isner and Roddick.
Isner on grass is very questionable against top players. Sure, he has had success in Newport, but that is no comparison. Honestly, Isner is much more dangerous on clay than grass. Sure his serve works great on grass like it should, but his poor movement gets exposed more on grass than on clay.
Roddick I would say could have a surprise run especially since this is best of 3 sets and at Wimbledon where the British crowd loves Roddick and would really pull for him. Plus, despite going out in the 3rd round to Ferrer at Wimbledon, he actually has shown many glimpses of his old form at both Eastbourne and Wimbledon. He was a bit unfortunate not to run away with that match against Ferrer, and had he won the match I think he could have beaten Del Potro pretty easily. Only issue I have with Andy is that he might be exhausted..