As of date Cogent (AS174) still has not entered into a peering agreement with Hurricane Electric (AS6939), resulting in significantly less prefixes than most IPv6 transit providers will give you. I've compiled a list of prefixes that are missing in Cogent's table.
Most IPv6 transit providers will give you roughly 3500 prefixes, but Cogent only carries around 2500 prefixes.
If you want to be reachable from all over the world over IPv6, it's best to get a second and third IPv6 transit provider that give you a full IPv6 routing table. In other words, avoid having a Cogent-only network.
txt file: ipv6-prefixes-that-cogent-misses-as-of-27-Oct-2010.txt