When I create a new network with IPv6, it sets the netmask_ipv6 and cidr_v6 in the database to 120, which is the equivalent in address space of a /24 in IPv4. This should not be happening since the smallest subnet in IPv6 is theoretically a /64 (the last 64 bits are part of the host address).
Radvd (which is used by nova-network) also requires as /64 for autoconf at the client(instance) side.
Command used:
nova-manage network create novanet --fixed_range_v4=10.153.200.0/24 --fixed_range_v6=2001:6f8:1477:1111::/64 --num_networks=1 --network_size=250 --bridge=br100 --bridge_interface=eth0
I'm not saying smaller subnets shouldn't be possible, but if you specify a /64 subnet (or larger) it should remain so.
Extra: Milestone is current essex release in the ubuntu 12.04 repos