Brilliant on one hand, unethical on the other. By using their application, I consent they use my bandwidth to carry other people's phone calls?

Why not if it says so in the ToS? Or it doesn't?

I'm sure they've covered their bases. They're not negligent.

I think they only very recently added a checkbox to opt-out of being a supernode though, the lack of which I found rather annoying.

As an interesting aside, that's also why Skype claimed Windows Update caused their last major outage - they claim there was a bug that caused the P2P network self-healing code to work slowly, a larger-than-normal number of computers rebooted at the same time after patching and the P2P network took two days to recover from the "critical disruption".