In preparation for World IPv6 Day on 8 June 2011, I thought I’d check my connectivity..
I visited a test-ipv6 page .. but the page took too long to open, had broken images, timeouts and readiness score 1/10 !
So I changed my MTU to 1280 and tested again
(In linux you might type: /sbin/ifconfig eth0 mtu 1280)
I then found this relevant discussion: IPv6 and path MTU discovery black holes
UPDATE 4/6/2011: An MTU of 1492 also seems to work correctly in my setup (as @chatasos prefers http://goo.gl/KRgm5)