The new Shimano Ultegra FD-6700G front derailleur has a smokey grey anodized finish that sets it apart from the standard 6700 version. Compared to the 6600, the shape of this unit has been tweaked to minimize the need for trimming the front derailleur, and the spring is lighter for less push necessary at the lever. The links have been widened to make the shifter torsionally stiffer. All in all, the shifting will feel smoother, lighter, and more precise.
The cage design of the Ultegra FD-6700G is wider than the 7900 cage and is designed to work with the front STI shifter that still has an intermediate "trim" shift -- Dura-Ace doesn't have this feature anymore. Shimano says this front derailleur will work with older 6600 shifters, but considers it 'B' compatible- it works, but not perfectly.
The Ultegra FD-6700G front derailleur is designed for compact (110mm bolt circle diameter) and standard (130mm bcd) double cranksets. There are three different versions -- braze-on, 34.9mm clamp and 31.8/28.6mm clamp (which comes with a shim to reduce the diameter from the larger size to the smaller). It can handle a big/small ring differential of up to 16 teeth (think 34/50 chainrings).