Hi,
This is an observation about the latest version of Skype. When you leave Skype in the background, it consumes the battery very badly. in fact in few hours you would drain the battery. The reason i think, Skype keeps sending and receiving packets often enough that this causes the data connection to stay up all the time and prevent the phone to go in dorment mode and save battery life.
I think in order to avoid this issue, Skype maybe send packets less often. In CDMA the connection goes dorment after 10 seconds of inactivity so it has to be much higher than 10 seconds.