IPv6 is supported outside of CCA. The 7900 series phones also support IPv6, but the CP-500 and SPA515G don't, AFAIK.
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/firmware/8_5_2/english/release/notes/7900_852.html#wp159417
We, however, do not recommend this configuration since it hasn't been tested on our platform (UC500) with the various other applications (like CCA).
Thanks,
Marcos