Skype is a VoIP app and gets special privileges for that. It's just a plist entry of course, but apple is known to reject apps for claiming to be VoIP apps when they are in-fact not (like Sparrow - http://buzzingpixel.com/article/2012/03/sparrow-push).
Also, be mindful of what you ask for. Skype is a terrible battery drain. This could be because Skype just has some bugs or it could be because iOS relly sucks at dealing with background apps which would mean that GTalk would use as much battery as Skype.
