In the past two years I've seen a number of companies at the Mobile World Congress working on solar and wind powered GSM base station solutions targeted at countries where the power grid is unreliable and many base stations are powered by diesel generators. Looks like the industry is now slowly moving from the concept phase to practice.
Apart from environmental issues, diesel generators need fuel which is sometimes very difficult and expensive to get to rural areas. So if solar or wind power can partly or fully supply a GSM base station with power, that's good for the bottom line and for the environment as well.
Here's a link to a press release over at TeleGeography that Orange Dominicana has started rolling out solar powered base stations. The article concedes at the end that its only 30 base stations for now, but it's a start.
The press release doesn't mention which solution is being used. At the World Congress, I've seen VNL for example, that develops very low power GSM base stations with limited range and Flexenclosure who work together with, among others, Ericsson (see picture on the left).