Not sure what you mean by "screw carriers". Any rooted Android device also has full tethering and supports VOIP & SIP calls automatically when within range of known Wifi (or 3g if you want). I'm not too familiar with MSN or Skype, but I'm sure there are apps that automatically take advantage of those calls when on a wifi network too.

To my knowledge iPhones also let you use VOIP on wifi and tether (with a cable)?