Another thing that would be cool is to make a VoIP call into HE. Or have us accept one from HE over IPv6. Even more fun would be for HE to make a call to me and require that it be done over IPSec.
The only open source IPv6 enabled pbx I know of is FreeSWITCH (or Asterisk v6). And both of those seem more like a test of the application rather than the carrier. That aside, one of us could always setup an ipv6 conference server, you know, just for kicks.