Recently, Nokia has announced that they will integrate Skype into the Nokia N97. Reactions, obviously, have been mixed. But I think the trend is difficult to stop, if not on this device it will be on another or in another way entirely. Some network operators have responded by announcing that they are thinking about introducing special tariffs which would include VoIP. But there is one thing over the top VoIP (i.e. non-operator circuit switched voice) doesn't have today, and that is the possibility to ensure the quality of service (i.e. latency, delay and jitter) especially over the air interface.
However, with a bit of imagination it wouldn't be too difficult to set this up. Here's one example of how it could work: In tariffs that take VoIP into account, the network could establish a secondary PDP context (UMTS) or a dedicated bearer (LTE) when it detects IP traffic of VoIP applications. This prioritizes the voice IP packets over other IP packets in the data stream of the user and also over IP packets of other users. Most mobile network operators already have deep packet inspection devices in their networks for all sorts of things and these could easily do the job.
I think it's an interesting technical possibility, let's see if somebody picks it up and puts it into commercial reality.