Genuinely surprised people are saying that Federer will one more slam [srs]. I gave him one more because I'm his fan and it was more out of optimism than realism.
I'd say he has to have a very, very lucky draw to win another slam. His fitness levels have dropped tremendously in my opinion over the past year.
His fitness/ability to retrieve has definitely declined but realistically he'd only have to have a couple of things fall in place to have another shot at Wimbledon this year, for example.
If someone took care of Murray and Berdych (and even then if he had a good day, he could beat either) then he'd have a great chance.