All players must make themselves available for at least two Davis Cup/Fed Cup ties (since the last Olympics) in order to be eligible for the Olympics. (One of those ties must be in the last two years before the upcoming Olympics.)
DC gives points but no money.
If playing 2 ties before the Olympics is the rule then Federer, if he wants to play in London olympic, would have to play DC at least 2 times before as Murray and all who normally don't play DC. No?