It's great to finally see IPv6 support!  This should be a big win.

I'm running IPv6 at 2002:c6ca:19fb:1:21f:16ff:fe29:d04c (git commit d6615a5 on Linux).  The system has a NATted 1918 IPv4 address, two 6to4 IPv6 addresses, and a Teredo address.  For the 3 days and 1 hour it's been up, bitcoind has apparently never tried contacting or been contacted by another IPv6 node.  It's started as "bitcoind -printtoconsole" and only has rpcpassword set in bitcoin.conf.  getinfo reports 8 connections (to be expected because I don't have IPv4 port forwarding set up).  I've verified it is listening on [::]:8333 with netstat.

-blocknet is now apparently -onlynet, where the meaning has changed to only allow one address family rather than exclude it.  I tried starting bitcoind with -onlynet=ipv6 and get 4 connections total.  IPv6 does work in this case.  It's been 2.3 hours, and I'll let it run like this for the time being.

IMO, the availability of IPv6 on a machine, even via transition technologies like 6to4 or Teredo, should increase the number of usable peer connections that the Bitcoin client has at its disposal; IPv6 should open up new opportunities for direct peering that couldn't happen automatically behind NAT.  From my experience with stock settings in a situation like this, the IPv6 improvements did nothing over that 3-day period.  Maybe there's a way to program the client or tweak the protocol to encourage additional IPv6 connections (without excluding IPv4 entirely)?