Federer has absolutely no problem with clay. He did semis in the FO. He will win no matter the surface. Is I was the Swiss captain, I would choose surface according not to Federer's best, but according to #2 player's best. The doubles match could be unpredictable, so favoring their #2 player is a good idea. You've got the two singles Federer's points almost for sure, so, why choosing grass just because is Federer's best surface? They don't need to do that, and grass won't favour their #2 player against Australia. I think it will be on clay, too. Let's see.
I agree US has a nice (not easy) draw, specially when comparing with the crazy other half of the draw (Spain! Argentina! Australia! Switzerland! Sweden!... this Davis Cup will be very nice to watch). But look out, Chile won't be easy specially if they play on home. The Chilean team will be very solid playing on clay. And of course the French won't be much easier.
Argentina, I think, will play on home and choose surface on almost all rounds if I'm not mistaken. That would be a HUGE factor, as they will play on clay and will have the support of a fanatic crowd. There is a frenzy in Argentina for winning the Davis Cup. If they play local on clay, they will be very, very tough to defeat. I'll pay much attention to the Argentinians.
And for my own country, Spain has a very tough draw. The positive side is that my fellow Spaniards rise and plays their best against the biggest teams, while they seem to not concentrate properly when playing "middle teams". As usual, their toughest test will be if the rival team choose Grass as surface.