I couldn't tell you how long these features have been in Asterisk but I decided to give IPv6 and SIP/TCP a try in the latest 1.8 release. Basically it's a simple process. There are no special options required in the build, not that I could find anyway.
Once it's installed, there are two configuration options that need to be set. By default Asterisk will only listen on IPv4 and UDP so add these options in sip.conf: