I probably set it to 'log in automatically when skype starts', as that's how I usually set it when I start Skype on a previously unused computer. Then I make sure that Skype is connected (i.e. I'm marked as online). When I go offline Skype goes offline, and when I get online again Skype goes online.

I don't know if it polls or what, but I just now did a test and the moment I disconnected the network Skype went into offline mode, and when I reconnected it immediately put me online again. So far I haven't seen any battery penalty for this, so it may be that it's been implemented with proper dbus support. (If it checked every second then there would be a huge battery drain over night afaik)