A default Raspbian installation does not have IPv6 enabled. That's quite a pity in 2013, where IPv4 addresses will be running out shortly.
Have no fear, it's not too hard to enable it.
ipv6 to the file
/etc/modules (it should be on a separate line) and reboot your Pi.
If you don't want to reboot, enable it immediately by issuing
sudo insmod /lib/modules/`uname -r`/kernel/net/ipv6/ipv6.ko. Note that after a reboot, you'll have to re-enable it again, so just change
/etc/modules either way…