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:
Obviously those two can be used independently if you want to try just IPv6 or just TCP, but where's the fun in doing one thing at a time?
So far everything is working just like it used to. I only have one phone that supports IPv6 or TCP, a softphone for Android named Sipdroid. Hopefully more devices will add support in the future.