Skype uses a lot of power and they are aware of the problem:
http://community.skype.com/t5/Mac/Skype-heat-fan-high-cpu-usage-issues-in-Lion-OSX/td-p/41264/page/2 http://community.skype.com/t5/Windows-Phone-BETA/High-power-consumption/td-p/543825
http://phandroid.com/2011/12/13/skype-updated-with-enhanced-video-quality-power-consumption-improvements/
The best thing you can do right now is to uses the latest version available.