Please take a look at the Network Documentation Tool:
It's more than just IPAM, but it was designed with IPv6 in mind.
BTW, I just gave a lighting talk at the I2 Joint Techs meeting in Salt
Lake City this morning:
http://www.internet2.edu/presentations/jt2010feb/20100203-vincente.pdf
Feedback welcome.
Regards,
Carlos Vicente
University of Oregon