Fed is now using the new BLX racquet from Wilson (though some think that it is a paint job on the K-factor). Here is what Tennis Warehouse says about the new racquet:
Roger Federer's Six.One Tour gets updated with Wilson's BLX technology. Wilson has woven Basalt fibers in the construction of the racquet for what it calls "the most advanced composites in the industry." On court the BLX version of the Six.One Tour picks up where the KFactor version left off. With a very similar spec, the differences between the two are subtle. Our TW playtesters found the power level and control pretty much identical to the KFactor version with the main difference coming in feel. With Basalt in the construction, the BLX Six.One Tour is designed to resist vibration and offer a cleaner feel. Compared to the KFactor version, the Six.One Tour BLX feels slightly more dampened. As with previous versions, the racquet is incredibly solid from all areas of the court. The racquet's mass offers excellent plow through on everything from groundstrokes to serves. At net, there is a high level of control thanks to the solid response and solid bite from the stringbed. All in all an excellent choice for the advanced 5.0+ level player seeking the ultimate in control.
So, the BLX Six One is not much different from the K-factor Six One other than better dampening (should help with those back aches) and better feel (something that Fed is always striving towards).
Someone else mentioned that the BLX is a slightly longer frame, enhancing the sweet spot of the racquet and that should help Fed with his BH.
I personally think that he is using the new racquet and this is not a paint job.