In principle, Voice over IP (VoIP) protocols, like any other Internet protocol, are just data flowing through the Internet. Many services, such as iChat AV, MSN Messenger, or Skype, provide VoIP computer-to-computer services for free. (Well, the services themselves don't charge anything for it -- you still have to pay your ISP for the Internet bandwidth to carry the data...) In this respect, there'd be no intrinsic reason to expect that a WiFi-connected iPod Touch would be differentiated in any way from other Internet-connected computers.
VoIP can often start to cost extra money, though, when you move into Internet telephony. That is, placing a call between a computer and a traditional telephone or vice versa. (As well, some ISPs which also happen to be phone companies, might feel inclined to get into the Internet Telephony business themselves, and then start intentionally blocking or throttling any other Internet traffic flowing through their network that looks like it might be a stream from a competing VoIP protocol.)
VoIP can often start to cost extra money, though, when you move into Internet telephony. That is, placing a call between a computer and a traditional telephone or vice versa.
I purchased a Touchmods microphone and use their free Siphon VoIP application. They both work great. I signed up for a low-cost VoIP/SIP service and routinely use this setup to place calls from Japan to friends in the United States for just pennies a minute.
Any online (ideally Europe or ship to Europe without silly courier rate) suppliers of a microphone for v2.0 iPod Touch non jailbroke.. Does such a thing exist?? eBay did not yield with search "ipod touch" microphone.