I ported the uipv6 network stack from http://www.sics.se/contiki/ to the Arduino platform. This library works with the ENC28J60 based Ethernet Shield from Electronic Kits Zone (see http://www.ekitszone.com/Products/4-enc28j60-ethernet-shield-for-arduion.aspx) on the Arduino Duemilanove. The library with example code for an IPv6 capable telnet server and instructions can be found at my homepage at http://sites.google.com/site/ghoelzl/ipv6ethershield. So the board can be reached worldwide 
and integrated into a sensor network.
I wrote a simple IPv6 capable HTTP server for the ipv6EtherShield-library (see http://sites.google.com/site/ghoelzl/ipv6ethershield). The output on a browser looks like this:
You can find the code of the server at http://sites.google.com/site/ghoelzl/ipv6ethershield/ipv6_http_server.
Have fun! 
How can you reach it from worldwide?
The main advantage of IPv6 over IPv4 is the huge 128-bit wide address space. As an example you can have up to 2^80 devices with public IPv6 addresses at home in contrast to IPv4 where you usually only have one public address.There is no need for address/port-translation, just simply plug the Arduino(s) with ipv6EtherShield into your home network and have access to them from all over the world.
Great job, the telnet server is a very nice little addon there 