Currently Being ModeratedFeb 13, 2013 10:39 PM (in response to xyzzy)
It seems that some ethernet adapters (in my case two different USB ethernet adapters) loos IPv6 connectivity when the system goes to sleep, causing the adapters to permanently loose IPv6 connectivity until the next reboot. This seems to be specific to individual adapters (i.e., the connectivity is lost for each adapter if connected while the mac is going to sleep).
As a workaround I added a script that is executed on sleep (using sleepwatcher) that disables IPv6 on the interface. When the system wakes up I re-enable IPv6 and then the adapter seems to properly regain IPv6 connectvity.