Wether its skype themselves or the networks (in my opinion most likely it will be them) to charge first, it will come in due time. At the moment everyone has 3G enabled phones and devices but majority of ppl dont abuse it or use it at all for that matter.
When everyone starts using skype to make calls for free then the networks will dung out a type of charge or something.
All you can do for now is enjoy it while its still there for free!