Telcos are holding everyone's dingdong pretty tight in markets where they dominate. Again, terrible argument against Nokia buying Skype. Nokia is the most carrier independent manufacturer out there. Not that I think Nokia buying Skype would be good idea.
They can, but that would ruin the success of Skype, reduce its user base and therefore be a loss for those who bought it.
You see, the bad thing about Skype is not so much codecs used... It's the mafia-like "closed network" method; the same used by Facebook or Twitter, btw. The reason why Skype has such a massive user bas is that you cannot communicate with a Skype user without being on Skype yourself. The network's closed. No open protocols, standards, interoperability. So it grows and continues to grow because whenever one of two lovers is on Skype, the other one needs to join. And the more Skype users there are, the more "gravity" it gains... If 9 out of 10 friends use Skype, why wouldn't you?
So yes, Nokia could open up everything after they bought Skype. But that would be like buying an expensive car and replacing the engine with flower pots in order to make it a nonpolluting object. It's a theoretical possibility but it's not going to happen.