"Just to let everyone know. I am currently working on deploying IPv6 network infrastructure for Valve's network. This is not a promise that support is coming, but rather to point out that without this infrastructure it is very difficult to implement IPv6 support.
Until last year, none of our network equipment would support IPv6. Our network providers also don't have pervasive IPv6 support, at least not on par with IPv4. So there are some technical challenges that we face with regards to network resiliency, which makes it risky for us to use in a production environment.