Pestered Sprint for....quite some time. Apparently Spark is the key. Spark-enabled phones in Spark supported areas should be dual-stack. Unsure how things like Happy Eyeballs might work. Sadly, my phone isn't supported, my area isn't listed as having or to be getting Spark in the near-term so I can't test this from a store and the closest city to try and test is a 3 hour drive away.
Anyone here willing to try it? Easiest test IMO is just go to http://ip6.me and click the 'IPv6 only test' link since depending on how preference for one protocol over another is implemented might affect the test to the main page and end up using IPv4. If IPv6 isn't working you should get a server not found error (or the like). If it does work, an IPv6 address.