Nokia has taken an opportunity to go into some detail about the Lumia 925 and its antenna improvements, utilising the aluminium to wrap around the sides of the Windows Phone. The "ring" of metal acts as part of the phone's radio system, with stripes that separate the antennas (main antenna at the bottom with two more towards the top of the device) from the other sections of metal.
The blog post notes that whether the owner is on GSM, WCDMA or LTE, it will provide consistent performance, as well as adjusting the "balance of power" between the antennas depending on how the Lumia 925 is currently being held. Woah; crazy stuff. The end result is the same radio performance as one would expect from a wholly polycarbonate design, but with an interesting aluminium twist.
It's humorously noted that should a Lumia 925 user hold the phone with two hands covering each and every edge of the device, this will have a negative impact on the signal. But it's stated this is the case with every other mobile phone. So relax, folks. We're not going to see the repeat of what happened with a competitor product only to have Stephen Elop state that we're all holding the darn thing incorrectly.
Source: Nokia Conversations