Does Skype running in the background use more battery life than other background running iPhone apps?
Apple limits what can run in the background to ensure battery life is not dramatically reduced. Skype will be constrained to the same restrictions that all other App Store apps must follow.

Given this, it is fair to assume Skype uses no more than any other Internet telephony app running in the background on your iPhone.

Fraser Speirs's article may be helpful in better understanding how background processes work on your iPhone.