An IPv6 PTR record looks like a reversed, fully expanded address with dots between each nibble. The zone name is "ip6.arpa".
Here's the current PTR record for "2600::":
Code:
0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.6.2.ip6.arpa. 21599 IN PTR www.sprint.net.