Have you heard about support for IPv6 protocol for any Ethernet shield? As far as I know only IPv4 is supported. Maybe some one would like to write library?
There is no library, and there physically cannot be one. IPv4 is built into the Ethernet shield itself. (It has a Wiznet 510 chip.)
Shields based on the MicroChip ENC28J60 chip (such as those sold by Ekitszone and Nuelectronics) are suitable for the implementation of an IPv6 or dual IPv4/IPv6 stack. This approach requires the TCP state machine to be implemented in software and results in more complex arduino code.
Source: this tutorial. (You may find it useful, I'd recommenced reading it.)
I have been working on an IPv6 library for Arduino, called EtherSia, that works with the ENC28J60 chip, but with a lower overhead compared with libraries that use Contiki / uIP. It is comparable to the EtherCard library for IPv4.
While the Wiznet chips do not support IPv6 natively, they do have a MACRAW mode that allows you to send Layer 2 Ethernet frames directly. I have managed to get IPv6 working with the Wiznet W5100 and W5500 chips, which is what the official Arduino Ethernet Shield and Ethernet Shield 2 use.
So it is possible...