The internet is starting to use a significant part of the world's electrical power, and increasingly complex algorithms are driving our economy. Some blogs even have started to discuss software engineering and global warming.
As software engineers, our choices have impacts on the energy usage. Delphi/Win32 still uses 8-bit character sets, which is faster, uses less hardware and is therefore a greener choice than UTF-16 based platforms like Java or .net.
How important is this? It's not important, at all. Any improvement in a specific part of a program, that is usually irrelevant unless you improve it at least 10 times. What about rewriting your software for low energy devices? Also not a good choice, because the real environmental problem with computers is to produce them - so don't make your customers buy new computers and think you saved the world.
If you want to do something for the environment, remove your focus from technology and focus on how to solve end-user problems well. Much energy is wasted elsewhere because of bad software.