For microcontroller-based OpenSprinkler, I did some Googling, and found that the current Ethernet library (EtherCard) does not support IPv6, but another library (UIPEthernet) does. We do have plan to change the code over to use UIPEthernet, and at that point IPv6 will be possible.
For OSPi, this is just a question of whether RPi supports IPv6, which I assume it must be able to.