"We actually kept all options open from the start, running two separate parallel R&D development tracks; One focusing on direct drive and one on a geared solution. It soon became clear that if we wanted to meet the customers' expectations about lowest possible cost of energy and high business case certainty we needed a perfect combination of innovation and proven technology and so the choice could only be to go for a medium-speed drive-train solution," says Finn Strøm Madsen, President of Vestas Technology R&D on this particular design choice and concludes: "Offshore wind customers do not want new and untested solutions. They want reliability and business case certainty - and that is what the V164-7.0 MW gives them."
....
Construction of the first V164-7.0 MW prototypes is expected in Q4 2012. Serial production is set to begin in Q1 2015 provided a firm order backlog is in place to justify the substantial investment needed to pave the way for the V164-7.0 MW.

As had been rumored, they're backing down from the novel (and controversial)  four-stage gearbox of the just now in production V112.  They are now following the path of AREVA Multibrid, using some gear stages to reduce generator cost, weight and complexity when compared with direct drive (Siemens).

Notice that they are giving themselves a short but workable amount of time to use testing results from 2013 and onward before they fix the 1st gen serial production. (Too short time to serial production has been an historical problem within the industry, worsened in China.)

I don't know yet if they continue to use carbon in the spars, and permanent magnets in the generator, as in the V112.

"Life shrinks or expands in proportion to one's courage." - Anaïs Nin