Today, Fring (App Store Link) issued an update to its application that allows push notifications to be sent for incoming calls (as well as other IMs). This means that you can now get alerted to Skype or SIP calls even when the application isn’t open, you are on a call or the iPhone/iPod touch is asleep. Therefore, your iPod touch or Phone can act more like a true VoIP phone. Fring itself is a solid IM/Voice client, though a little rough around the edges.
Another nail in the telco’s coffin? We can only hope.