No. The Skype version for Maemo is, as far as I can tell (by look&feel) a recompile of a Linux version for the desktop (an earlier one than the one I currently have on my actual desktop). It works just the same way.

When idle, no. I've left Skype active & connected for hours through a tethered phone, watching the amount of data through that 'connection administration' tool (which shows the amount of data sent&received), and although there is some traffic it only accumulates to some hundred kilobytes after a couple of hours. In this mode I haven't been voice-talking to anyone, only using chat now and then.

Disclaimer: All of the above is of course with the version of Skype that is on my N800, not the N900 version.

EDIT: As to P2P networking, I'm quite certain that this version *does* work the same way as the desktop version, otherwise it wouldn't be able to work its way through firewalls the way it does.
My guess is that it will never go into supernode mode though (because that would easily suck a lot of data through the connection).