For VPN purposes, a throwaway Jabber account you use solely for that is perfectly acceptable though. The thing needed here is mostly a channel for two hosts to find their public IP addresses and communicate that information.
p2pvpn <http://www.p2pvpn.org/> uses BitTorrent trackers for it - which is actually a pretty good solution, but sadly also - no NAT holepunching or even UPnP yet.
Though I have been trying to find a VPN which uses a Tor hidden service to define network rendeavous point - since Tor is distributed and available, you could issue invites with the hidden service ID, and then send real IPs to members to establish P2P (so, not using the anonymity, just using it to bootstrap the network). SocialVPN does something similar, but I couldn't get it work reliably in tests (it would go up...then my hosts went down and I couldn't get them to appear to each other again).
