Anything can be challenged with something BETTER. Look at me...
My first IM was ICQ. It was the best thing around, as it was the first IM.
Then, MS Messenger became stronger, and I ditched ICQ.
Then came Skype and my MS Messenger was history.
All ditches "losers" I mentioned were leaders once, brand names, top of their game. You think I wouldn't switch to the next best thing if it came tomorrow? Sure I would, in a heartbeat, especially because it IS an instant messenger, all my friends will switch to something better. I don't want to stick around talking to myself...
The questions you really need to ask is:
1) How can a competitor improve over what Skype has to offer now? What would you have to receive as a user from someone else to just say: "Wow, I gotta move!"
2) Do I have what it takes to challenge them?
So to sum it up:
Can Skype be challenges? Yes, Of course. Question only is with WHAT an by WHOM.